I just noticed this problem in the last month or so. I was not sure what was happening so it me a while to catch on. At random intervals, the HD Display Windows Audio driver stops seeing my preamps. The device will not reappear if I power cycle the preamp but a reboot of Windows does fix the problem.
Initially I was using an Anthem AVM 70 preamp and I discovered that the Anthem room calibration software (ARC) would still talk to the preamp when in disappeared from Windows Audio device list so apparently the HDMI connection was still working. But I would have expected power cycling the AVM 70 to be sufficient for the device to reappear in Windows.
A few days ago I swapped out the AvM 70 to send it back to Anthem for a warranty issue. My dealer loaned my an NAD T187 preamp, which is about 12 years old, as a replacement. Today I was playing some music using JRiver and once again the audio device disappeared (this time in the middle of a song). So it seems like there is something special about the HDMI connection to my HTPC, but I can’t decide what the next step should be.