@arnaudrigole Please ensure that if you have STP enabled on this “test” port, you must ensure one of the fast STP functions are enabled either RSTP, Port Fast, or Fast STP (depending on what your building switch maker calls it). Or just disable STP on the “test” port to see if that fixes your problem. I would suggest leaving STP on and making sure one of the fast protocols are enabled.
We have also seen GreenEthernet (802.1az) cause problems with the FOS engine.
Out of curiosity, what make and model is your building switch?