Same here... about 8 years with brett and no cross contamination issues. Buckets are sanitized with hot water, oxy, and star-san, and glass for secondary. Plastic carboys for sacch only, since they can’t take the heat. But you’re just as likely to get cross contamination from saison yeast as brett, in my opinion.
I agree, it’s probably some other piece of equipment... hose, airlock, stopper... it’s easy to miss a piece when sanitizing.