I see you mention a bit later that these devices are also connected to the network via a wireless interface. I would have suspicions that one or some of the below issues could be in play:
(1) The devices establish independent bridged segments across the LAN over the wired interfaces and wireless interfaces.
(2) On one or both of those segments, certain L2 traffic such as BPDUs, UDLD PDUs, etc., are discarded -- either unidirectionally or bidirectionally. In particular if BPDUs are discarded across a segment, any loop created through that segment will not be detected by spanning tree.
It may even be as simple as the endpoints themselves forming a wireless bridge between one another, and then the wired side of >= 2 endpoints completes the loop into the LAN -- where the wireless segment passes all traffic except for BPDUs.
This can be a bit difficult to detect if any time the problem is introduced, the loop renders the network into a non functional state. However, you may be able to infer the above potential issues if trouble is seen only when both wired and wireless interfaces are active together -- but not when one or the other is inactive.