mine cart apparently "spings" out of the rail.

... as a result, it looks like I'm never there right now.

I empty a Nether fortress with mine carts. The mechanics of the mine carts are known to be unpredictable. So I have to run regular patrol to dissolve mine carts, there are wedged.
Sometimes is a mine cart by side to the rail (see picture), even on a straight line (no picture taken).

It's definitely a bug and not a result of the physics of the mine carts