VLAN 1 is the default PVID as you have not specified anything different.
/interface bridge port add bridge=vlan-bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus1 pvid=1 # implict default PVID
/interface bridge port add bridge=vlan-bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus3 pvid=1 # implict default PVID
The setting is irrelevant as you have frame-types=admit-only-vlan-tagged
On a Mikrotik a bridge is treated as a separate subsystem, it has an implicit bridge-to-CPU interface so bridge traffic can access other services within the device but it is perfectly possible to configure a bridge which has no interaction with them. This may help http://forum.mikrotik.com/t/routeros-bridge-mysteries-explained/147832/1