Bug seems mostly fixed. However, in 1.14.4 pre release 2, mobs still experience the spinning if they walk on the top half of a waterlogged slab (top of block is slab, bottom is water).
[media]I've found that this bug also occurs when mobs try to walk on halfslabs that directly at water level (the lower 'half' of the block is filled with water while the top is slab). Really sucks for my slab-based raft village. I'm not sure if it's related, but creepers that blow up on these blocks also don't destroy any blocks, as if they were in the water (but they are not).
Made a separate ticket for the bug pertaining to the top halves of slabs (thanks Fabian) : https://bugs.mojang.com/browse/MC-156022