Hit a really weird bug with the light-weight multi-selection on iPad:
Long tap one item with finger A to start dragging: fine. Tap another with finger B: fine, gets added to my stack of dragged items. Tap a third with finger B: 💥 Triggers primary action while the other two remain in dragging state. What the... Huh? Why?
Works fine in another app though. And in the app where it doesn't work, it works fine when using my Magic Keyboard and holding command and tapping all the items.
Now to comment out random blocks of code to figure out what's interfering here...
Got it! There was another gesture recogniser and its shouldRequireFailureOf delegate method was only getting called on the first two taps, but not the last one. Seems like a UIKit bug, but I'll work around it.