Summoning a slime with levitation, so that they aren't affected by gravity, won't reset the slimes AI, so they'll move in whatever direction they were last moving indefinitely. Because of MC-87573, a slime summoned with levitation will move south. All other mobs won't move at all when they have levitation.

Steps to reproduce:
1. Run

/summon Slime ~ ~5 ~ {ActiveEffects:[{Duration:300,Id:25,Amplifier:1,Ambient:1b}]}

2. Observe the slime drift south.

Other details:
-This happens even when specifying a rotation value of 180, the slime will turn to face south. Again, this is because of MC-87573.

/summon Slime ~ ~1 ~ {ActiveEffects:[{Duration:300,Id:25,Amplifier:1,Ambient:1b}],Rotation:[180.0f,0.0f]}

-If you give levitation to a pre-summoned slime the slime will drift in the direction it was facing.

/effect @e[type=Slime] levitation 100 1

-This works across all levels of levitation, but most of my testing was done with level 255.