Files with AC3, DTS and DTS-HD would not play if audio-spdif=ac3,dts AND device is not S/PDIF (S/PDIF override issue)

Important Information

Provide following Information:

• mpv version mpv-x86_64-20201206-git-a9aa5a1
  EDIT: BUG STILL IN mpv-x86_64-20201220-git-dde0189 !
• Windows Version Windows 10 Home x64 2004 (b19041.685)
• Source of the mpv binary Shinchiro mpv-x86_64-20201206-git-a9aa5a1.7z
• If known which version of mpv introduced the problem: Shinchiro 20200906-git-f57b90b
• Possible screenshot or video of visual glitches : N/A

Reproduction steps

1. Edit mpv.conf
2. Add inside it:
   ao=wasapi
   audio-spdif=ac3,dts
3. Do not use an S/PDIF device: select an analog output. Example: Speakers (you can do this test in ANY computer)
4. drop a video with AC3, DTS or DTS-HD stream (I don't have others, but I guess EAC3 and others could fail as well)

Expected behavior

It is expected that the file plays.
It is expected that audio-spdif have no effect because it detects that the device is not S/PDIF.

Actual behavior

It remains stuck at 00:00:00.000. You can have it unstuck by simply seeking forward. After that, playing continues smoothly.

Things to know:
With the exact same config and files, if you select an S/PDIF device, everything works smoothly, no bug
If you remove audio-spdif=ac3,dts, everything will work smoothly, no bug (and no S/PDIF obviously).
So it is the combination of audio-spdif=ac3,dts with NO S/PDIF device that is causing this issue.

With the exact same config and files, if you use an older version (before 20200906), everything works smoothly, no bug, in all the cases mentioned above.

Log file

Here is a log where I drop the file and keep it stuck 00:00:00.000 (meaning: I did not attempt to seek), then I close mpv:
output with no seek.zip

Here is a log where I drop the file, it's stuck at 00:00:00.000, then I seek forward a bit, it's unstuck and the file is playing. Then I close mpv:
output with seek.zip

Sample files

No need, any standard file with AC3, DTS, DTS-HD streams.
It is possible also E-AC3 and True-HD show same behavior, but I have none of that kind to test.

Thank you!!

Start by reproducing with ao=wasapi and audio-spdif=ac3,dts on an analog Speakers device, using the linked logs to compare the stuck and seek-unblocked cases. Done means AC3, DTS, or DTS-HD playback starts normally without seeking when no S/PDIF device is selected.