If STP[1] is enabled, but that is unlikely since you'd have dropped connections when roaming for the reasons you just gave. Most likely, STP is not enabled on these networks.
When a switch has learned a mac address all traffic destined to that traffic would be immediately switched to that port. If the switch has no record for that specific mac address it floods all ports except the ingress port. This is expensive and means other devices receive traffic that isn't intended for them so they waste time dropping it.
So in networks that have no protections against those attacks then this could very well be a problem if there are multiple access points and the two nodes are on different access points.