In pre-release 8 all of my flushing type mobs farms have stopped producing. Mobs are still spawning, but when the water flushing starts they either hop up and down in place or stand around like they're in a childrens wading pool. There is little to no movement.
These same farms were working as expected in 1.13-pre5 through 7.
Occasionally some will move a little, and if enough mobs accumulate then some will get pushed into the trap due to collision boxes, but all mob farms that rely on flushing are effectively broken. My iron farm, creeper farm, darkroom spawner and guardian farm all no longer function.
And to be clear, I'm not talking about vertical movement in water/air mobivators, I know those are broken, and it's intended (due to the new rules for the undead and drowned mobs).
I'm talking about horizontal movement in flowing water. Items still go with the flow, but mobs are barely affected.
I noticed this issue with my iron farm as well, getting awful iron drop rates since upgrading to pre-release 8