That’s ok! After downloading a Subtitle editor, which by the way,
couldn’t change the default flag. I got all absorbed with what I ELSE
I do with subtitles! Wow, that’s a deep “Rabbit-Hole”.
I begin to understand your interest better!
I made some formatting changes that could not be saved as .srt
and .MP4 won’t accept .SSA files, so…
I’m back to .MKV, while not fully supported by Windows (why?) it is a
MUCH more versatile format, while MP4 can only have one audio track
and one subtitle track, the subtitle can only be added during encoding.
Muxing extra tracks into a .MKV is easy!

Now I’m wanting to back and re-do a film I subtitled last night,
but THAT encode took 6 hours! I guess that will be a .MKV too! 😉

Cheers!