Unfortunately I don't have any answers, but some observations. I'm running 8.5.0.1. I'm on the road, but still have a decent amount of access to my lab. I went to check and sure enough, I do see an SSID advertised from my mesh network. The interesting thing, is the SSID is not the MSSID that is created in the mesh profile. The SSID is the mesh cluster profile name itself, not the cluster name that is assigned with the profile (which should be the actual MSSID value, at least that is my undertanding). Here is my mesh profile
ap mesh-cluster-profile "mesh1-cluster"
cluster "mesh1-mssid"
wpa-passphrase <psk>
opmode wpa2-psk-aes
According to the CLI reference guide the "cluster" command sets the SSID, as stated in the docs "Indicates the mesh cluster name. The name can have a maximum of 32 characters and is used as the MSSID for the mesh cluster."
I did try to connect to this from, and of course was not able to. I also have a mesh running on my 6.x network, and cannot see the SSID at all. If I recall correctly, in 6.x, the MSSID was part of an information element in the 802.11 frame.
I hope this helps,