The cart I was on wasn't moving and had just gotten pushed off the rails. The rails are laid on a single layer of nether blocks with nothing below it. This one layer bridge is a completely enclosed tunnel with a dimension of 2 wide and 3 tall (if that makes a difference). When getting out of the mine cart I clipped through the blocks and fell into the lava below. Will be putting a second layer layer or catching system below in the future but this doesn't seem right.

This issue seemed similar to a resolved problem with this number attached:

1. MC-60284