This bug, previously reported by me, still exists in sddm 0.15.0-1 Additional information:
The problem (black/blank screen) exhibits only when connected via an A/V receiver. Sddm displays normally when attached directly to the TV (4k). NVidia softare correctly enumerates the A/V device, so the latter must be sending correct EDID info. Also, it reports 1080p resolution, which is the maximum input resolution for the A/V device. The problem clearly lies with sddm. BIOS displays correctly, grub boot menu displays correctly, and the KDE desktop displays correctly. Only sddm fails to display correctly, though it did display correctly when previously using a legacy NVidia device on the same system. -- "The world is a dangerous place to live -- not because of the people who are evil but because of the people who don't do anything about it." --Albert Einstein