I am working on a diagonal track (left/right/left/right).  I place the minecart on a curved track and use the minecart. This places me on the right side of the minecart.

When I reach straight track, I take damage from the lava behind the obsidian wall.

There is a junction ahead (right/left/left/right, keeping me on the same z=-108 line) and after those curves, I am centered in the minecart.

This happened 3 of 3 times that I placed the minecart somewhere on that diagonal/curves.