Dog upper lip changed colour after boarding??

I boarded my dogs at a “nice” place but it’s been nothing but problems since getting them back. Will never do that again 🤦🏻‍♀️

My male dog came back with horrible diarrhea that was so bad his butt was leaking all day. I posted about it here and everyone thought it might be his anal glands but the vet didn’t think so and so we’re just treating the diarrhea as stress induced because his fecal test came back normal. He’s on special diet and probiotics and it seems to have helped and he’s almost back to normal.

The other thing I asked the vet about was that his upper lip was red. The vet thought it was irritation from licking his hind end while dealing with the diarrhea issues.

It’s been 5 days since I got him back and it actually looks like his upper lip has lost pigment. What could cause this???

First pic is before boarding and second is now.

He isn’t licking excessively or anything other than a few licks of his legs or paws here and there but I’m not sure if I should go back to the vet again about the lip.