Myztero - bug or not?

Skadis debuf is no status element but a stack. You can’t cleanse it, block it and mysterio can’t remember it. So that’s safe.

For Liz. I mean even if mysterio would remember it, nothing would happen.

He always remembers effects exactly as they are.
When he remembers some 4 turn poison damage without further restrictions and you cast him he does exactly that, 4 turn poison damage.
If you remember tellurias 2 turn manaslowdown he does exactly that, 2 turn manaslowdown.

So now he would remember liz manaslowdown. That reads like that:

So the duration is no number of turns but the presence of the fiend.
So if mysterio would remember the manaslowdown from Liz and you fire him, the opponents would get the manaslowdown, there are no fiends so the duration would end instant.

It doesn’t make any difference if he remembers it or not. There wouldn’t be a manaslowdown at the opponent. The only difference would be, that one of the 5 slots would be blocked by a manaslowdown without effect on the opponent.