… and, with the benefit of my face in the code, it turns out that this was just because I completely misunderstood what that button was supposed to do!
That one (unlike the timeline scissors) doesn’t split at just an arbitrary frame on the timeline - it splits the text on the text cursor in the text edit box in that widget as well. So you can use it to split the subtitle at a selected part in the original text instead of just duplicating all of it. You need to have the timeline cursor set to where you want the clip cut and the text edit box cursor set to where you want the text split, then that button will be enabled.
I’ll add a patch to my current stack to try and clarify the help text for it.