The manual, as OP said, does not offer any explanation, why the device might show up with an additional MAC/IP at the upstream switch port, and which services i

The manual, as OP said, does not offer any explanation, why the device might show up with an additional MAC/IP at the upstream switch port, and which services it might offer. OP sounds knowledgeable enough to be able to exclude the possibility, that the additional MAC/IP could be from one of the PCs, like e.g. when playing with VMs using an internal bridge in the Hypervisor. Maybe the device has a bigger "cousin" device, that includes "control via APP", and this feature was not properly/fully disabled on this one.