I am still a bit confused, never mind. I did put out an update fixing the enemies showing the wrong lose scene, try updating to the latest version.
Just checked and is still there. I'll try to explain it as clear as possible: let's say that an enemy has 1) sprite; 2) attack animation; 3) attack pattern (speed and damage the enemy does); and 4) s*x animation.
On lvl 4 all anemies from all previous levels appear, even bosses. It's here where the problem is. The level 3 boss (the plant looking one) uses the sprite and attack animation from the level 1 boss (the troll with a stick) but the attack pattern and s*x animation are from the level 3 boss.
Note: now I'm too strong to lose so I can't check the s*x animation that plays there, so maybe now it uses the one from the troll as well.