The management traffic is tagged on a trunk port. I mentioned RSTP, LLDP as they are vlan independent.
If you require to have management traffic egressing untagged, port must be configured for hybrid mode. Please note that a port in hybrid mode is not automatically added to every VLANs present in the switch, and user must configure the hybrid port in selected VLANs. Please refer to this link for explanations of different port mode supported on cnmatrix :