However, , the more normal Tinderbox approach to this sort of import would be to use the Explode feature: see Exploding Notes. That avpids needing to use the split function.
My test looks sutup selection was like this, and it worked.
Depending how clean the source is, in terms of sections starting with the note-to-be title’s text, you ought to get a very clean extract to note in one explode. Note you may find it easier to make some light editing of the source if you don’t get the titles you want.
It shouldn’t impede here as Markdown rendering is off by default. But, if you are a Markdown aficionado, I suggest you don’t turn on the Markdown support until you’ve done the explode and lost all those #### character runs which look like Markdown H4 markers! Different people do very different things in Tinderbox so not all defaults suit all users, q.v. the prototype/location auto-detection described above.
Also, Tinderbox generally expects to work with smallish notes in terms of amount of text. There isn’t a limit but the fact selecting your test TBX’s ‘Part 61’ note is quite sluggish suggests this is an excess. so, once the import is done I’d either delete the $Text of 'Part 61" (first saving somewhere outside Tinderbox if still needed as one text). Or, after moving out the explode-generated notes from under ‘Part 61’ just delete the note. Look up the aTbFef notes above on Explode to see the purpose of the ‘exploded notes’ sub–container created by an Explode.
If the shortcut doesn’t work it is likely another app has ‘taken’ that shortcut. This occurs because (to the best of my understanding) when an app starts it signals to the OS the shortcuts it needs. So, after a macOS session boots up, it is first-come first serve. As munmu-bar apps are often opened as post start up they are the best pace to look. For instance I use to have a problem with Browse Links shortcut until I discovered 1Password also mapped it and 1Password always opened first.
Surprisingly, it there is no simple Mac report that tell us which shortcut is mapped to which open app.
So fixing the issue, it is a two step process. Find the app already using that shortcut and if possible delete or re-map the shortcut. Otherwise close it and re-open Tinderbox before re-opening the other app. Or make a custom OS shortcut to use a different shortcut to call Tinderbox’s split.
It has always seemed that OS X / macOS should have a shortcut-conflict discovery app, because obviously the OS knows about the conflicts, but keeps it secret. KeyCue can help with discovery, but not with conflicts. ShortcutDetective is excellent, and slowly dying.