Yes, as someone who ships a lot overseas (from Germany no less), I can confirm that this happens all the time when you use regular registered mail rather than a courier service.
@Luthien, you could try plugging the Deutsche Post tracking number into the Canada Post tracker (rather than using the Deutsche Post website), and sometimes that works, but in my experience it's only really USPS this works for.
I'm also not sure that the employees in small-town Canada will even understand the Rückschein part, as that's just not a thing that exists in the Canadian postal system, so I wouldn't worry too much if you don't get that or it's delayed. This is actually the exact reason why we couriered ours -- I wasn't worried about speed so much, I was worried about not being able to track when it got there. That said, the odds of it
not making it there are very slim, so you're right that it's almost certainly fine. (To be honest, I've never had Canada Post lose anything, whereas Deutsche Post has somehow lost two important sets of untracked documents for me, so if you tracked it until it left the country you should be fine, especially if you indicated it only contained documents so it doesn't get held up at customs.)
As for the medical appointment, you already know all about our troubles dealing with it here, so I can sympathize!