I wear my pump (a Ping) in my bra frequently and have yet to have an issue with it malfunctioning while next to my skin - and I live in Central Florida where during the summer we often have 90+ degree temps and 100% humidity!
I’m small chested so I can position the pump in such a way that it provides a little ‘boost’ (if you will). I usually put it in horizontally, so that one part is leaning toward my skin while the other part acts as kind of a…plate I guess you could say? The angle makes all the difference, when I don’t put it in the bra that way it does end up looking like a weird middle boob. :-p