Sorry, I just saw this and another thread about not showing subtitles in mkv files but sam nazarko pointed some user to this thread about 3 days ago. Here is said thread:
https://discourse.osmc.tv/t/local-subtitles-not-displayed-while-playing-mkv-file/
True, I was on alpha 4, did not know about RC release.
I will do a fresh install today as suggested elsewhere, thanks for pointing it out!
Thanks DBMandrake!