when I leave a Minecart for a long time circling and next to it there are more rails but a lower block goes to the lower rails and goes back up

What I expected to happen was...:
That the Minecart that I put stays where it should

What actually happened was...:
That the Minecart that I put up went down the rails below

Steps to Reproduce:

1. Make a square with rails, place a Minecart, make it move and a block under that square of rails forms another square of rails
2. stay away from the place until the chunks are not loaded
3. and comes back