MT doesn’t know that untagged frames, entering through ether1 (and wlan1) are considered as “native VLAN 8” on other devices in LAN, those frames are simply untagged. You’re tagging them on ingress (PVID setting on corresponding bridge ports under /interface bridge port) and untagging on egress (untagged members of corresponding VLAN under /interface bridge vlan). But as you tag frames on ingress through e.g. ether1, those frames are tagged inside bridge (the switch-like personality) and, if PVID is not set on bridge interface, they are expected to be tagged on bridge interface (read more about bridge personalities) … Only that bridge interface in your case is not member of VID 8 in any way (neither untagged nor tagged), bridge interface has implicit default setting of pvid=1 which means that default mikrotik configuration has VLAN ID 1 as “native”. If you want to have VID=8 native, you have to reconfigure ROS in 3 places.
To better understand how VLANs are done in ROS, read through this excellent tutorial.
As I wrote: as long as bridge doesn’t have vlan-filtering=yes, it won’t do tagging/untagging and untagged frames from switches will indeed get to bridge interface untagged and things seem to work fine. If you enable vlan-filtering, then you’ll loose management access to device (you’ll get upset and consider things not working and revert the setting). Unless you fix the bridge interface vlan settings.