it would be worthwhile looking at whether a pH drop of 0.04 has that much effect on contrast. You could try a few test clips all with the same time but with pH varied through 8.1, 8.14, 8.18, 8.22.

If you see a big difference, it might be worth investigating what Xtol does differently. You could also try and compare buffer strength of Xtol vs. your dev, which can be done by preparing a small quantity of both, measuring their pH, then putting same amount of acid into the liquids, then measuring pH again.