Disabling PRIME rendering does fix the issue -independent of the setting of "adjust refresh rate"
In my case it did not help (LE 11 Nightly 20210828), in advance after disabling PRIME rendering and reenabling the audio stopped working - from the Audio output devices all vc4-hdmi devices disappeared and it was necessary to reboot LE to get them back (restarting Kodi did not help).
Gave it another try as well with the 10.0.0 release. Subs didn't work at first like before, disabling PRIME rending did fix the issue for me however and subs are now working fine
That doesnt seem to be "the" solution either, because now x265 videos are stuttering.
So I turned PRIME Rendering on again, x265 video then played fine, but the ass subs were gone. Then I changed the render method from direct-to-plane to EGL and this "fixed" the ass subs, but now x265 videos are completely black with just the audio playing.
Tried disabling the hardware acceleration for DRM Prime and now both seem to be working - ass subs and x265 videos, but cant yet tell if x265 videos are stuttering, will have to watch a movie for that
Edit: Well that doesnt work either. Now x265 HDR content isnt shown at all. All I see is the loading icon and hear the sound being played, pressing back gets me back to the interface/menu with the sound playing in the background, in both render methods.
So if I want to watch something where I want the ass subs I have to turn off hardware acceleration, and for anything else I have to turn it back on basically... Other than that annoying bit, the new LibreElec works great 
