I suspect that the worthiness enchantment that ODIN placed upon it is malfunctioning.
The hammer was destroyed yet again in the sun and THOR eventually reassembled it, but what if he missed a piece?
The hammer has been damaged and destroyed in the past and reassembled with no problems of side effects, such was when THOR shattered it blasting into the Celestial's brain vault only for them to put it back together for him.
When the Destroyer first appeared it damaged the hammer also.
When THOR ended the Ragnarok cycle wasn't the hammer briefly destroyed and then reassembled also?
I never objected to Steve Rogers being worthy, he makes sense. But otherwise, no one else exept for THOR, BRB and Rogers. Also Dargo from the future.
Hmmmmm, perhaps a storyline where Dargo comes to the past and meets THOR , perhaps aiding him with his hammer problems?
Just a guess but I think there is something wrong with all of Asgard and mjolnir is the most obvious clue. That is based on Thors' comments.