Before i start, I understand that this bug is quite similar to MC-59509, however that report pertains to a different version of minecraft entirely, and has different requirements to execute.

I was trying out flying machines with chest minecarts being trapped inside.

what i expected to happen: the track would break when the block beneath was moved, trapping the minecart.

what actually happened: the track began dropping itself as an item without ever breaking.

to replicate:
1. place 3 slime blocks in a horizontal straight line.
2. place a minecart on a track in the middle
3. cover the remaining slime with any movable block (i use iron)
4. attach an engine and move the entire contraption in a straight line.

note: it is important that nothing is directly pushing the iron blocks besides the slime directly below it. see pictures for more details.