I am trying to give slow falling to a Zombie. In the past, I just used level 250; for some reason this works fine for players but not for mobs? They still move upwards no matter what level I put, as long as the level is valid (Levels above 255 or below 0 are invalid).
Currently I have:
/effect @e[type=zombie] 25 100 250
But this just makes them fly upwards extremely fast.
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So how do I get slow fall/negative levitation?
You could update to 1.14 and give entities the slow falling effect.
Edit: Or update to 1.13.

1.14 sucks. Its too much like a mod now, with all the tables and whatever. 1.13 was ok, because oceans WERE pretty empty...
Slow falling exists in 1.13 as well.
Either way. Neither effect amplifiers over the default maximum value nor 1.13 are supported by Mojang.

Ok, I'll word it this way. What value of levitation will cause mobs to fall slowly. Anything between 250 and the max value of 255 works for the player, but what value will do this to mobs?
We are volunteers that handle bug reports, not questions about things that are not supported. Try searching somewhere else.

So what you're saying is NO value of levitation will cause slow falling for a mob?
What we're saying is: Any level of levitation that is over 4 is not supported and behaviour is nondeterministic with such levels.
We currently only accept bugs in 1.14. Also, effects over the max default value is not supported.