... I rather stick with a preset value
My point is that on a Canon body, by selecting Flash as a WB setting, you're not
selecting a fixed preset value, just a modified form of Auto WB.
I wouldn't call this a 'bug' yet. Granted, it should be a fixed color temp with something other than a Canon flash in the hotshoe - but PWs do strange things sometimes, and I bet if you send it to Canon they'll not find an issue.
BTW, what makes you think the issue is with the body and not one of the monolights?