On some occasions, the slime mob would launch me several dozen blocks in the air rather than knocked back.

What I expected to happen was for the slime to just knock me back like usual and like other mobs do.

What actually happened was on a rare occasion a (medium sized has only shown this happening) slime would fling me very high in the air when he hit me in mid air from jumping.

I have a few different occasions this happened with different conditions. For each occasion I was wearing gold armor with status effects from these commands:
/effect @p minecraft:jump_boost 999999 255 true
/effect @p minecraft:saturation 999999 255 true

Steps to Reproduce:
Occasion 1:
1. Spawn slime (size:1 or medium sized has shown to work)
2. Continuously jump and let the slime hit you (Slime can't always hit you)
3. Make sure there aren't any blocks above you as it doesn't seem to occur then
4. If it doesn't work, try against a tall flat wall and let the slime come after you

Occasion 2:
1. Exactly the same as Occasion 1, but with these additional status effects:
/effect @p minecraft:resistance 999999 4 true
/effect @p minecraft:weakness 999999 255 true
2. I was also holding an enchanted gold sword with Protection IV and the Unbreakable NBT tag when these status effects were present

This is all that I have been able to find about this. It is not easy to repeat, but has happened multiple times with similar conditions. It seems to only happen with the medium sized slime, but that doesn't mean an other size/type can't do the same.