I tested with the last server release 188.8.131.52 and app 1.7.60 and it's becoming worse with the test file. The subtitles are gone now completly. I sent a report and the logged in user was Michael.
This time I set the subtitles to keep the formate and accept the transcoding for a test only and the subs were there. Unfortunately is my NAS not powerful enough to keep this setting.
I tried the same file with emby on my iPad and there was all perfect, furthermore i could keep the ass subs styled. Seems that the LibMpv Video Player is more capable than the exoplayer.
How does the web app compare?