I was using Arc Pro 3.3.0 then patched up to 3.3.1 but this issue persists. When classifying data using Graduated Colors Symbology, no matter the method, e.g. Natural Breaks, Quantiles, etc, at some point the Classes box just resets itself to 1 class. I enter a different number, hit enter, and it reverts to 1 class. The workaround is to pick a different symbology mode, like Bivariate Colors, which resets everything, then go back into Graduated Colors, reselect the appropriate field, enter the number of classes, and it works. For awhile. . . Not the end of the world, but annoying and unnecessary side trips and clicks for the functionality to work correctly.
