Vlan_filtering + STP not working


I am pretty desperate right now.

I have TP-Link Archer C6U v1 (EU) routers.

With vlan_filtering 0, everything is OK.

As soon as I activate vlan_filtering (1), STP is not working anymore and a patch between LAN2 and LAN4 ports creates a loop.
I am also seeing the STP packets leaving (untagged) on LAN2 and LAN4 interfaces.

The error:
br-x: received packet on lan2 with own address as source address (addr:xx:xx:xx:xx:46:2f, vlan:2)


config device                                   
        option type 'bridge'                    
        option name 'br-x'                      
        option vlan_filtering '1'               
        option stp '1'                          
        option forward_delay '4'                
        option igmp_snooping '1'                
        option priority '4096'                  
        list ports 'lan2'
        list ports 'lan4'

config bridge-vlan
        option device 'br-x'
        option vlan '2'
        list ports 'lan2:t'
        list ports 'lan4:t'
        option local '1'         

config interface 'lan'
        option device 'br-x.2'
        option proto 'static'
        option ipaddr ''

Thank you very much!