Creating a bubble stream in a 3x3 waterfall (source blocks only at the top) using soul sand or magma causes new water source blocks to appear directly inside the stream. I am not sure if this is intended behaviour, but it does seem quite suspicious.
How to re-create
1. Create a 3x3 waterfall by only placing water source blocks at the top.
2. Put a piece of soul sand/magma in the middle of the waterfall's bottom and wait for the bubble stream to appear
3. Remove the magma/soul sand and the previously placed water source blocks at the top of the waterfall.
It should then be very obvious that new water sources were created.