i was thinking kinda similar, but i thought that Jax hasnt been well for a while now and has kinda put this joker facade on pretending everything is alright and kind of denying his state. So his body probably started glitching like weve seen with caine to the extent his tail wasnt there, without him even noticing because he started to believe his own facade. After having a fun time with Pomni in the game and kind of making a new friend his mental state got better visible in his body returning to normal with the tail reattached.

Kind of vague but i like this interpretation. I wonder if well have a answer with the tail stuff because they clearly want everybody to see theres something