The flowing lava goes up but the source is down ! Take a look :

How to reproduce it : place a lava source, place 2 block further and 1 block lower a lava source, please be careful that the lava flows only in the direction of the other lava source. There is an example on the 3rd and 4th screenshot.

Expected behavior : the flowing lava should not connect to the top source because of the 2 ends of the 2 blocks.

Observed behavior : the flowing lava connects to the source of the top when it should not because of the 2 ends of the 2 blocks that separates the flowing lava from the bottom and the source from the top.