I was trying to make lavacasts on a test world and this happened. The water straight up replaces the lava.

1. Steps to reproduce:
2. Make a pillar of nonflammable blocks at least 10 blocks tall
3. place lava on top block and wait for it to fall to the ground 
4. place water on lava sourceblock. This should turn the four surrounding lava spaces to cobblestone
5. place water on top of the four cobblestone blocks 

Expected results: The lava would turn to cobblestone as the water flows down and comes in contact with it, creating a lavacast/cobblestone tower. 

Observed results: The water flows inward towards the central pillar and replaces the lava. It’s somewhat hard to describe but if you reproduce this you (hopefully) will get what I’m saying.