This occurs in the new Minecart Experiment

How to reproduce:

There are two tracks perpendicular to each other, but one track is raised above the other. See image below:

If a minecart were on the raised track and would reach the raised intersection, it would be expected that the minecart would fall onto the track below. Indeed, this is what happens normally. However, if there were a minecart already present on the bottom track, the first minecart would land on top of it. Then, the second minecart would be pushed away even if it didn't have motion. This results in the first minecart freezing mid air:

The problem is NOT that the minecart is stuck on the block for the raised track. Removing that block does not fix the minecart:

Watch the attached video below for a full walkthrough of this bug:

8mb.video-4qE-UVCxngQA.mp4

Other observations

Relogging into the world fixes it. Nudge the minecart slightly also works, or you could break the lower track.