We bridged a V5.5 link using station psuedobridge mode (yes, I know but it was unavoidable for this particular application) and standard 802.11 protocol. A Windows laptop connected to the bridged Ethernet port pulled an IP from DHCP server on the AP Wlan without an issue. A Linux machine on the same Ethernet port will not pull an IP. This was tested with various laptops, wireless cards, routerboards, AP’s, modes, etc.
We rolled back to version 4.17 and everything worked as expected.
If you have this issue, 4.17 may be your solution.
Issue reported to support.