There are several possible reasons for this:

1. We don't know the full capabilities of the Time stone / Amulet of Agamotto. When fighting Dormamu he needs to reset time in an entire dimension, in another he just needs to reset time in one locality or for one being. We see him doing the same thing with the apple in the Doctor Strange movie. In Infinity War we also see him using the stone to send his consciousness to review possible futures. The stone appears to have many time-related capabilities, not just one.

2. He's fighting Dormamu in a dimension unlike ours, which does not have time. Perhaps the stone operates differently there. There he is able to reset the situation between himself and Dormamu but both remember what happened before. He is able to use this to "bully" Dormamu into abandoning his plans.

Personally I favor the first explanation.

There are several possible reasons for this:

1. We don't know the full capabilities of the Time stone / Amulet of Agamotto. When fighting Dormamu he needs to reset time in an entire dimension, in another he just needs to reset time in one locality or for one being. We see him doing the same thing with the apple in the Doctor Strange movie. In Infinity War we also see him using the stone to send his consciousness to review possible futures. The stone appears to have many time-related capabilities, not just one.

2. He's fighting Dormamu in a dimension unlike ours, which does not have time. Perhaps the stone operates differently there. There he is able to reset the situation between himself and Dormamu but both remember what happened before. He is able to use this to "bully" Dormamu into abandoning his plans.

Personally I favor the first explanation.