Looking for some advice/confirmation of my understanding on some network issues I'm having.
Have you tried configuring bonjour?
https://documentation.meraki.com/MR/Client_Addressing_and_Bridging/Bonjour_Forwarding
When clients connect on Mx ssid and AP ssid are they in same subnet? Do you know that there is no roamming between Mx wifi and APs ?
Roaming is a client dicision. So you can still roam if the ssid is the same and the AP ssid are in bridge mode and using same vlan.
The wireless function of the Z-Series and MX is designed to act as a standalone infrastructure base station in a small office or home office (SOHO) site.
There is no roaming between MX-xxW and MR, because they don't exchange any information about the client database, security, etc. Roaming is more than just going from one access point to another.
There's a lot of ambiguity here.
Some questions that need to be answered
If the answer is no to both of the above. Then your focus right now should be on layer 3 roadblocks such as firewall rules, VLANs and NAT instead of Client roaming behavior and L2 mDNS/bonjour.
Do you receive the same IP address when connected to the GR10 that you do when connected to the MX65W?