Apologies if this is already an issue. I did search for minecart void with no results.

I just upgraded my server to version 1.15, went to my old school endermite enderman farm via a ~150 block long, 2 high minecart tunnel for repairing.

The minecart track has a path the whole length of the minecart track, 3 blocks underneath, except for the killing platform which is only a single layer above the void (the platform is at level 4) and a normal powered rail stop block with a button at the end on the same killing platform.

When exiting the minecart in any direction at the killing platform (ie single block layer above void) I fall straight through the block and into the void. I would expect to stay on the block layer beside the track.

This is repeatable in my multiplayer world. Loaded a backup and the exact same thing happened.
It did not happen in 1.14.x or earlier. The farm and track to it was built at least 2.5 years ago and has never had the issue before.

The attached image is just of the platform itself...it's just there floating a couple of blocks above the void.
(that farm still works well enough with entity cramming too )