With these times and mixed a week ago I cannot see how it can be a developing issue. What the OP doesn't say but we are inferring, rightly or wrongly is that he has used Xtol at these times in the past with TMax 100 and Ilford Pan without the problem.
Maybe he can say if the combination of Xtol and Tmax is new to him. Has he tried to do any darkroom prints and if so are the negs a problem to print?
I had my first experience of TMax 400 this week and with Xtol 1+1 at 9.25 mins(official Kodak time) the negs looked thin and very grey compared to both Ilford and Fuji Acros films but the prints were fine.
It is the prints that count and until an attempt at printing has been made then I don't believe the OP can conclude that there is a problem
We need to know more before we can help him or decide if indeed he needs any help