In Thor: Love and Thunder, Mjølnir
reassembles itself for Jane. Exactly why is hard to tell. The movie implies that it did so because Thor accidently programs it with the purpose to protect her, however, this scene may instead be explaining how Jane was able to wield it at all (thus meaning she wasn't worthy in Odin's eyes).
If it gained the ability of reassembly because of Thor's reprogramming, then the question of why Odin didn't give it that ability to begin with arises. If it already had the reassembly ability, then the question of why it didn't reassemble earlier arises.
So, why/how did this happen?