I do this with my OTTB. He doesn’t stop at first fences but he has certainly stuttered and/or jumped from a standstill at first fences when not on the contact. Enters the ring like a giraffe who just had an edible. Lots and lots of leg. I work harder entering the ring–getting him round and getting him to give me an ear flick to let me know he’s heard me–more than any other time on course.
There is also merit to jumping it at a standstill vs. circling and going back to it. It’s not hard for them. It’s also not what I’d call fun for them. They do learn it’s more fun and easier to just canter it instead.
To me it sounds like he’s learned he can take a minute to get his bearings and it’s fine, and that behavior needs to be replaced with, “if there’s a jump in front of me, I should jump it because otherwise my rider will make me jump this from a standstill”***
***barring all medical reasons being ruled out.