When breeding axolotls with tropical fish or a bucket of tropical fish, there is a roughly 10-second pause where the two axolotls will simply hang motionless in the water with all their limbs in the default position. I guess this is the axolotl equivalent of a T-pose.

During this time no particles appear and if the axolotls are bumped by any other mob or the player, they will fail to breed and revert to their normal behavior. The rest of the game seemingly continues to operate as normal.

The problem is illustrated in the attached video. I don't have a specific explanation, but it appears similar to the issue in early 1.15 snapshots where bee AI would stop and they would float motionless until bumped.