YOU ARE A GENIOUS!
It does makes sense because when you have a target locked, we force the character's rotation in C++, when the target is not locked, we just update her rotation when she attacks (to make she face where the camera is aiming at before attacking). So maybe the ragdoll effect that happens when you receive that charging or horn attack from the boss is causing conflicts with this rotation we set with lock-on, causing the capsule to get under the ground collision. If this is the problem, we can solve it by simply disabling the lock-on rotation when the player is under ragdoll effect and BOOM!
I'll try this solution after I'm done with a new female\female animation and let you know if this was in fact the problem. Thank you!
