Hello Hias,
thank you very much for the solution, it fixed the problem.
Enabling "Adjust refresh rate" seems to even have removed some minor stuttering I noticed in some video's, which I always assumed were caused by the RPi's hardware being limited, or my network drive. So, two birds with one stone 
Thanks again,
Ksarijoop
I can confirm that this also doesn't work in LibreElec 9.97.1 on an RPi4. Several x265 files with embedded .ass subs didn't show any subs, even though the subs were recognized. Switching to EGL rendering only shows the subs, not the video (black screen). Disabling PRIME rendering does give correct subs, although the CPU had some trouble keeping up.
The same files played fine on LibreElec 9.2.6.
