if you are booking the ‘one way’ as YUL-EWR and EWR-CLT as separate flights, then yes, could be trouble. If you are booking a ‘one way’ by using multi-city YUL-EWR, EWR-CLT,, then it will all be on one ticket and you’ll be fine and protects you, and then doing the CLT-YUL on a different PNR, you should also be fine, presuming it’s not something like an immediate turn.

Speaking of one ways and Canada, at least from my experience flying to YYZ, assuming the one way fare is half the round trip (which I typically see these days, although noticed it wasn’t the case on my last trip) it is typically cheaper to book one ways. I believe it’s a tax thing - not sure if it’s just this or also something else also, but you only pay the Canadian HST on the trip from Canada if booking as one ways, but pay the HST on the total round trip fare if booking as a round trip (or multi city) US-Canada-US. I find it saves about $35/person on my typical trips, so when traveling with the family (which is most of the time), the savings are worth it.