A trick I do once in a while is to do /export verbose, then send it to a file. I can then grep (search) the file to see if there is something I missed. Try that while looking for "stp" maybe...
If you provide a packet trace showing the STP frame with your mac address as the source, plus /export to support they would have to look at it.
Might as well upgrade to 6.41.2 to make sure the problem still exists in current.