Mister Hatt wrote:3GP is not the same as MP4

They are interchangeable for a certain range of features, of which Timed Text is one.
Which is irrelevant, because it's simply a renamed MP4 file to get QuickTime to play ball (which only acknowledges text streams when the extension is 3gp, similar to how it only recognizes multiple audio streams when it's .m4v).
Mister Hatt wrote:as I recall you had some issues getting ffmpeg to play nice, which seems to go along more with what I said...
Oh look, you're just making shit up again.
Seriously, do your research and quit wasting my time with this crap.