I asked a CM yesterday and she said there was no grace period but she didn’t sound very sure of her answer.
Today when we went to ride at about 9:15 (I believe) it had just broken down so i asked if we still had to be back within our window (we had an hour left) and the CM told me we didn’t because it broke down but that it had to be relatively close, he said we couldn’t come back and expect to ride in the afternoon for example. We went and used a fast pass and got some food and came back at 11:30 and were able to ride no problem even though our window was up at 10:15.