when you place lava on a flat area, it spreads in all directions like normal.
it flows 3 blocks out. when you dig a hole, and place lava close to it, it will only spread towards that hole, as normal. however, it even does that if the hole is 4 blocks away from the lava. So it cant reach the hole, but still flows towards it.

How to reproduce:
dig a hole in a flatworld
place lava 4 blocks away from it