You have to cancel each ADR yourself. Resort reservations and ADRs are independent of each other.
BTW, it takes only a few minutes, at most, to do the cancelling. Just go to MDE on the app or your computer and there's a Cancel button next to each ADR.
A CM explained it to me that it's because you might have just gotten a last minute offer to stay with friends who live in the area, and it would be rather upsetting if Disney cancelled your ADRs.