Reported to pulseaudio as: With Linux 5.5 kernel, pulseaudio seems to be unable to recognise a present nvidia hdmi output (#802) · Issues · PulseAudio / pulseaudio · GitLab
This started happening with Linux 5.5, with both older patched nvidia drivers and with 440.59. It seems to be due to changes in the audio driver to support displayport MST and probably isn’t directly dependent on the video driver, but I couldn’t say that for sure.