I read a lot about XTOL on APUG from the usual suspects.

Let's start with the KODAK XTOL Data Sheet.

The experts say, keep air out, with full to the top bottles, floating lids, inert gas, N2 Burst in deep tanks.

and replenished aged XTOL may last years to be the BEST XTOL!

However KODAK allows inversion, rolling tanks, open trays, deep tank Dip & Dunk, N2 Burst Tanks, all which I would think could add a lot of air over time to any replenished developer.

Perhaps a chemist can explain how large area air exposed XTOL fluid surfaces with agitation waves, remain air free...

Over time...over months...

Or is simple evaporation the real problem?

I don't know the answer.