Ime, players are entirely willing to accept an extremely short session just so I can prep and set back up after they throw me a massive curveball. If you're capable of doing it on the fly, that's great, but I'm not and my players usually understand.
Had a twelve minute session once because I forgot I gave the party a foldable boat like three months ago on a whim, and they used it to skip the next ~3 sessions of content. I had an entire thing setup where they'd help a dwarfhold hunt a dragon, and had started on some city-based intrigue in the next area.
I just leveled with them that I had not even slightly expected this session to go this way and had nothing prepped so we'd stop early and pick it up next time.
I hate this take a lot, I'm gonna be honest. I don't care if his game is so on rails that it's set on the fucking orient express. As long as the players are having fun with the game, and the GM is having fun with the game.. that's a good GM.