Hello there, @rexj ...
I've not seen this behavior before (to be honest, I hadn't tested anything like your scenario before)...but what you describe is interesting. I could see why one would expect the migrated quizzes to appear in a module. That would make sense from a course design/layout standpoint. On the flip side, however, I can also see why New Quizzes wouldn't be automatically populated into the module. I assume/imagine it could be because New Quizzes are technically brand new items in the course that get added to your Quizzes index page, and Canvas doesn't necessarily know where to put them (although, one would assume that they should be put in the same module that they were originally added to). I know it's not ideal to re-add them, but when you click the "+" sign to add items to your module, you can just Ctrl+click on each New Quiz in the list to add multiple quizzes all at the same time instead of one-by-one. That might save you a little bit of time.
Not sure if any of this is helpful or not, but thought I'd share my initial thoughts. Be well.
