It appears that, as per the discussion in this thread ( https://lore.kernel.org/linux-pci/CAJZ5v0j9GS2y0tpnzaGu8n9=kbhd9qkbudguancjz01u+px...@mail.gmail.com/T/#u ), a commit ( https://github.com/torvalds/linux/commit/4d6035f9bf4ea12776322746a216e856dfe46698 ) and merge ( https://github.com/torvalds/linux/commit/7266f2030eb0e17c442aa6cf527263828eed79bd ) in 5.13 was made reverting the PCI power management change which caused the issue.
I would assume the revert will be pulled into the next 5.10.x release. I would suggest this revert be included in the Debian 5.10 kernel for release, given that it breaks (at least one) system(s) and the old (proven) method did not.
