[HELP] after encoding ass into mp4, but there is no subtitle at all

[Selur]

the original MKV is blueray remux here, and why you have no problem with chapters there?

Yes. (only difference seems to be, I a. use way slower encoding settings b. I use closed gops)

btw. does remuxing the mp4 with mkvtoolnix to an .mkv fix the playback issue?

Also looking at the debug output, you are using an old Hybrid version (btw. 2021.02.27.1), may be it's a problem with that version or one of the tools used in it which has ben fixed for month,...