If you push a waterlogged block with a piston, the water disappears from it after being pushed, which is why I believe the behaviour described below is a bug.

When you instead of pushing a waterlogged block do a "block drop" on it with a sticky piston - that is to extend and retract a sticky piston within less than 3 game ticks, so that the block that is attached to it remains in the place where it was when piston was extended - the waterlogged block keeps water inside. This behavior is easily visible in the gif I linked below.

https://gfycat.com/BeneficialQualifiedGlowworm