In the second to last sequence of this epic Throne Room battle, Rey is seemingly disemboweled by a crested Praetorian Guard wielding a seperateable melee weapon called
vibro-arbir blades. But she screams "No!" and the blade does nothing and then vanishes. Far from being a cinematographical error, this must be the old "use the force to protect yourself from a blade and then make him fling it off camera" move.
This fight is isolated here in a loop:
https://screenrant.com/last-jedis-throne-room-continuity-error-doesnt-matter/
Regarding Luke's metal hand disappearing with him, it's clearly because it has become part of his body and one like his bones. This is pretty easy to think through, unlike the fight scene discussed above. Simply put, it must be some awesome force-move of Rey implied.
