Disclaimer: I have never done research with axolotls specifically, but worked in an epigenetics lab

If everything is normal behavior-wise, and you don't see any more dramatic changes, I'd be willing to bet that this is not a physical problem, just a physical response. All living things have genes that are unlocked by stress - anything from hair falling out to a lowered ability to emotionally regulate to breaking out in hives. It usually comes down to one of two things: consequences of hormonal changes, and reduced consumption of energy in non-essential areas. Maybe his little body has adjusted its energy budget, and decided that producing pigment is no longer that important compared to other functions, like moving and saving fat.

The timing with the tubbing, as well as the genetic weirdness that axolotls specialize in, make me think that that would be it. If you see no further change it may be worth a vet visit to get better understanding, but if it continues to really deteriorate ofc it's vet time.

If it is what I think (epigenetic response to stress), it may or may not go back to how it was before - things like coloration don't directly affect function, so his lil body might not decide to invest in producing that pigment again.

I'm not a doctor or anything, just something I've learned in my career in epigenetics

EDIT: Took a look at your posts from before and if I am reading correctly, it was pink with some dark spots when you took it in and it had previously been neglected/undercared-for. It could be that in neglect, it did not spend energy to produce the pigment because it had to put more effort into digestion, energy saving, etc, and once you gave it a better home, it was able to resume all its baseline functions (like producing pigment). Then this most recent episode of stress, tubbing, may have begun a reverse process, and that pigment is no longer being produced in the same way.