I have a Lenovo IdeaPad 5 with and AMD Ryzen 5 and I have the same issue:
stefano@stefano-IdeaPad-5-15ARE05:~$ lspci -nnk
00:00.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Renoir Root 
Complex [1022:1630]
        Subsystem: Advanced Micro Devices, Inc. [AMD] Renoir Root Complex 
[1022:1630]
00:00.2 IOMMU [0806]: Advanced Micro Devices, Inc. [AMD] Renoir IOMMU 
[1022:1631]
        Subsystem: Advanced Micro Devices, Inc. [AMD] Renoir IOMMU [1022:1631]
00:01.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Renoir PCIe 
Dummy Host Bridge [1022:1632]
00:02.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Renoir PCIe 
Dummy Host Bridge [1022:1632]
00:02.2 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Renoir PCIe GPP 
Bridge [1022:1634]
        Kernel driver in use: pcieport
00:02.4 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Renoir PCIe GPP 
Bridge [1022:1634]
        Kernel driver in use: pcieport
00:08.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Renoir PCIe 
Dummy Host Bridge [1022:1632]
00:08.1 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Renoir Internal 
PCIe GPP Bridge to Bus [1022:1635]
        Kernel driver in use: pcieport
00:08.2 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Renoir Internal 
PCIe GPP Bridge to Bus [1022:1635]
        Kernel driver in use: pcieport
00:14.0 SMBus [0c05]: Advanced Micro Devices, Inc. [AMD] FCH SMBus Controller 
[1022:790b] (rev 51)
        Subsystem: Lenovo FCH SMBus Controller [17aa:3829]
        Kernel driver in use: piix4_smbus
        Kernel modules: i2c_piix4, sp5100_tco
00:14.3 ISA bridge [0601]: Advanced Micro Devices, Inc. [AMD] FCH LPC Bridge 
[1022:790e] (rev 51)
        Subsystem: Lenovo FCH LPC Bridge [17aa:3828]
00:18.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Renoir Device 
24: Function 0 [1022:1448]
00:18.1 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Renoir Device 
24: Function 1 [1022:1449]
00:18.2 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Renoir Device 
24: Function 2 [1022:144a]
00:18.3 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Renoir Device 
24: Function 3 [1022:144b]
        Kernel driver in use: k10temp
        Kernel modules: k10temp
00:18.4 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Renoir Device 
24: Function 4 [1022:144c]
00:18.5 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Renoir Device 
24: Function 5 [1022:144d]
00:18.6 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Renoir Device 
24: Function 6 [1022:144e]
00:18.7 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Renoir Device 
24: Function 7 [1022:144f]
01:00.0 Network controller [0280]: Intel Corporation Wi-Fi 6 AX200 [8086:2723] 
(rev 1a)
        Subsystem: Intel Corporation Wi-Fi 6 AX200 [8086:0080]
        Kernel driver in use: iwlwifi
        Kernel modules: iwlwifi
02:00.0 Non-Volatile memory controller [0108]: SK hynix Device [1c5c:1339]
        Subsystem: SK hynix Device [1c5c:0000]
        Kernel driver in use: nvme
        Kernel modules: nvme
03:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. 
[AMD/ATI] Renoir [1002:1636] (rev ce)
        Subsystem: Lenovo Renoir [17aa:381b]
        Kernel driver in use: amdgpu
        Kernel modules: amdgpu
03:00.1 Audio device [0403]: Advanced Micro Devices, Inc. [AMD/ATI] Device 
[1002:1637]
        Subsystem: Lenovo Device [17aa:381e]
        Kernel driver in use: snd_hda_intel
        Kernel modules: snd_hda_intel
03:00.2 Encryption controller [1080]: Advanced Micro Devices, Inc. [AMD] Family 
17h (Models 10h-1fh) Platform Security Processor [1022:15df]
        Subsystem: Lenovo Family 17h (Models 10h-1fh) Platform Security 
Processor [17aa:381f]
        Kernel driver in use: ccp
        Kernel modules: ccp
03:00.3 USB controller [0c03]: Advanced Micro Devices, Inc. [AMD] Renoir USB 
3.1 [1022:1639]
        Subsystem: Advanced Micro Devices, Inc. [AMD] Renoir USB 3.1 [1022:1639]
        Kernel driver in use: xhci_hcd
03:00.4 USB controller [0c03]: Advanced Micro Devices, Inc. [AMD] Renoir USB 
3.1 [1022:1639]
        Subsystem: Advanced Micro Devices, Inc. [AMD] Renoir USB 3.1 [1022:1639]
        Kernel driver in use: xhci_hcd
03:00.5 Multimedia controller [0480]: Advanced Micro Devices, Inc. [AMD] 
Raven/Raven2/FireFlight/Renoir Audio Processor [1022:15e2] (rev 01)
        Subsystem: Lenovo Raven/Raven2/FireFlight/Renoir Audio Processor 
[17aa:3821]
        Kernel modules: snd_pci_acp3x, snd_rn_pci_acp3x
03:00.6 Audio device [0403]: Advanced Micro Devices, Inc. [AMD] Family 17h 
(Models 10h-1fh) HD Audio Controller [1022:15e3]
        Subsystem: Lenovo Family 17h (Models 10h-1fh) HD Audio Controller 
[17aa:3820]
        Kernel driver in use: snd_hda_intel
        Kernel modules: snd_hda_intel
04:00.0 SATA controller [0106]: Advanced Micro Devices, Inc. [AMD] FCH SATA 
Controller [AHCI mode] [1022:7901] (rev 81)
        Subsystem: Advanced Micro Devices, Inc. [AMD] FCH SATA Controller [AHCI 
mode] [1022:7901]
        Kernel driver in use: ahci
        Kernel modules: ahci
04:00.1 SATA controller [0106]: Advanced Micro Devices, Inc. [AMD] FCH SATA 
Controller [AHCI mode] [1022:7901] (rev 81)
        Subsystem: Advanced Micro Devices, Inc. [AMD] FCH SATA Controller [AHCI 
mode] [1022:7901]
        Kernel driver in use: ahci
        Kernel modules: ahci

Brightness key are working and the system brightness slider changes, but
no real brightness change happens. Brightness is not changing even by
using the system brightness slider. This was lastly verified with Ubuntu
20.04 and kernel 5.4.0-54-generic.

This issue is temporary solved by the following action
- Compiling amdgpu.ko module from a 5.7.0+ kernel (I used 5.9.8 kernel) and 
manually substituting to 5.4.0 module
- Installing a new BIOS update
but after a shutdown and power on of the PC (if you restart without the PC 
powering off before turning on again this doesn't happen) the problem comes 
back again. The only permanent solution I found is to compile a 5.7.0+ kernel 
(I am using 5.9.8 kernel right now:

stefano@stefano-IdeaPad-5-15ARE05:~$ uname -a
Linux stefano-IdeaPad-5-15ARE05 5.9.8 #1 SMP Mon Nov 16 19:58:12 CET 2020 
x86_64 x86_64 x86_64 GNU/Linux

and I can always use brightness normally.

Please implement new kernel correction for this issue into Ubuntu kernel
please!

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https://bugs.launchpad.net/bugs/1829641

Title:
  brightness keys on laptop don't dim the backlight

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