I have the same problem with hyperion on an RPi4 2GB. I first experienced this on my RPi3+ and thought it was an performance issue. I tried again with my new Pi4 this morning and the flickering still appears. It might be noteworthy that although the screen goes black, the ambilight seems to be working just fine. Without hyperion the playback is working fine. With all other sources, local media, zattoo, mediathek, there is no problem at all.
I can give more details on my setup and provide logs as early as Saturday.