Bridge has multiple personalities, very well explained in this article. The traffic stats is about the “the switch-facing interface of the router” … and when other devices communicate with each other and router only acts as a switch, then counters will not increase.
Or is it about the “the router-facing port of the virtual switch”? But the stats will be pretty static in this case as well.
Quite a few settings in /interface/bridge are about “the router-facing port of the virtual switch” … and implicit default setting for pvid on any of bridge ports (including the before mentioned bridge facing switch port) is PVID=1. The only way of getting rid of this setting entirely (on trunk ports) is to set the port to “frame-types=admit-only-vlan-tagged” … and this as well applies also to “the router-facing port of the virtual switch” … in which case PVID setting (either implicit default or explicitly set) is ignored and port is not added as untagged port to VID in question.