Actually the production of the platy butter without inbreeding tends to indicate that the platy combo does not involve a recessive gene.
Hard to be sure but I would also not guess that it's recessive since lesser X lesser has not been reported to produce platy.
One theory is that a lesser or butter is turned into a platy by the addition of a single copy of another gene in the lesser/mojave/butter/Vin Russo/phantom/"whatever makes a crystal" group. However it's different from the others in that just one copy of the gene apparently makes something normal or pretty much normal looking and the same for two copies if that combination is viable. I like the name dilute for this gene because it sounds like a rat gene I once read about that was described as having no effect by it's self with the normal types but dilutes certain morphs. However, given the lack of platy from lesser X lesser it seems likely that the gene is linked to lesser, perhaps by being yet another version of this same gene (or maybe even the lack of that gene) so that none of the lesser offspring of platy X normal would get it but all of the non lesser offspring would.