Transmission seems to be slipping, but smoothly?

This is a 2001 Ford Escort with automatic transmission. The transmission was actually rebuilt just about a year ago by a very legit place that takes care of their customers. Their reviews are amazing and they go out of their way to do stuff that most places don't, so I don't think they did anything wrong. It worked fine for a year.

It is kind of like a slight jerk, but not rough. It is more of a smooth slipping kind of jerk. It sounds as if it doesn't want to change gears properly and sometimes sounds like it stays in the lower gear when it should have switched over. Then later on, eventually it sort of slips into gear with a faint slip/jerk.

Any ideas? Fluid is full. Just started doing it recently. Surely it couldn't have worn out that quickly.
Added (1). I think the warranty is over after a year or a certain amount of mileage. Pretty sure that's up. Not because the shop tried to cheat us out of it or anything, but because it has simply passed the amount of time and mileage.

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