Precisely. Same issue here. The fact that the TX audio "comes back" when WSJT-X is restarted seems to strongly suggest the issue is with WSJT-X.
I have the same issue with no transmit audio. This occurs after my
screen power setting turns off my display. Upon turning the display
back on, no transmit audio is sent.
Once this happens, I can restart WSJT to fix it. Or change the TX sound
card to a wrong value and then change it back to the correct value.
Interesting once I fix it this way, further screen power being turned
off does not cause the problem.
This has to be a WSJT bug.