Water Does Not Fill Manually On Top of Other Water Blocks

Normally, water blocks placed with a bucket should (usually) merge with another water block two blocks away to fill in the empty one in between and form a new water source block. In this snapshot, this works on top of solid blocks, but no longer works when done on top of other water source blocks. 

Discovered in survival mode on top of water also placed by a bucket.

ETA Confirmed in creative.