The bug
When pushing a minecart through soul sand or mud, it passes through rails and will not attach to them.
How to reproduce
Place some rails on soulsand
Get a minecart on top of the soul sand near the rails (for example by temporarily placing a rail with the minecart and breaking the rail)
Try to push the minecart ontop of the rails
→ ❌ The minecart does not "snap" onto the rails or try to move over them
Affects 1.16.2 Pre-release 1