If you download it with a separate SRT, and merge it yourself, does it work correctly?
MrGrackle
If I use “Extract Original Format” (+ manually convert to srt) or
“Extract to SRT File” and merge with current MKVToolNix GUI
the result is/was always correct.
I rechecked using 6235: Forced Subs with Remux Into File
are now correct with both mkv (ffmpeg + mkvtoolnix)
but with Video Format mp4 it's still wrong. (like post #2+4 pic)