When exploding tnt lava doesn't update

To reproduce:
1. Just light some tnt on the lava y-coordinate.
2. Now there should appear some not working lava source blocks in the air fi you exploded enough tnt.

Issue:
Lava is misbehaving when large quantities of tnt explode next to it. Instead of flowing away on all sides of the source block, it hovers midair.

Expected results:
The lava should hover in the air for a moment and then start flowing down on the 4 sides and the bottom of the source block.