In L2.5 bridging, the Client Bridge inserts its MAC address in the source
MAC field of any frame that passes through it. This essentially
'hides' the MAC address of the client device from the rest of
the network.

Can prevent applications using MAC registration.
In a network where
users must authenticate, such as a hotspot location, once one user has
authenticated, all users behind that client bridge may be able to use the
network.

Traffic originating from behind one client bridge may have intermittent or
no communication with devices behind other client bridges on the same
wireless network.
If multiple client bridges are used in one
'chain‘, that client bridge then connects via Ethernet to
another AP. That AP has one or more client bridges, network communications
in either direction may be intermittent or non-existent.

See below:

This is an ARP request. It’s destination layer 2 address is the
entire broadcast domain and it’s layer two source address is
it’s wired NIC card. As you can see it knows the
destination’s IP address (layer 3), but not the
192.168.1.90’s MAC address.

Below is a reply to the request.

Download arp-nat.pdf for more info.