Even trying to add the effect using /entitydata (E.G: /entitydata @e[type=Zombie] {ActiveEffects:[{Id:2,Amplifier:100,Duration:300,Ambience:0}]} ) does not work.
The only way to apply slowness to a mob it's by throwing potion.
Added a new situation when the bug appears and added a forced crash report.
Confirmed for 1.13.2