There are two different functions here:
- Tupletise existing notes: this works outside of step entry mode - you select a group of notes, then hit the tuplet shortcut. It assumes the rhythmic value based on the first note selected. It works reliably as long as you’ve selected a group of actual notes. It works less reliably if there are rests involved, as you’ve discovered. By the way, plenty of other people have discovered this, too, and reported it.
- Create new tuplet. Only works within step entry. The rhythmic value is reliably determined by typing it before invoking the popover.
I think you’re getting these confused. They’re both very helpful functions, but they do work in slightly different ways.