This happens when the Minecart Improvements experiment is enabled. Mobs standing with their hitbox flush with the rails will not get picked up by minecarts like they used to. Instead, they have to have their hitbox overlap with the area of the rails. This was not a mentioned change in the changelog.

Without the experiment, mobs can also be picked up through distances as thick as 3 block pixels (a trapdoor's thickness) from the rails, but now, they cannot.