Hi.
We had exactly the same issue some weeks ago and MX64W.
SAP spend 3-4 days on this without any look. We did realise the connection to the firmware upgrade.
We finally rollback to 14:53 (from 15:42 release candidate that Meraki had push out) and now everything works again.
Then we had a control upgrade to 15:42 one week later to find the root cause.
The problem is not that everything works at 15:42. So now we don't know what and why. really a strange cases.
I have my personal opinion and that is firmware upgrade didn't finish properly the first time. I know that version 14:53 sam some bad bugs where firmware upgrade can hang.
It is not so useful but interesting to see that others have the same problem with SAB B1 on separated VLAN as Client
Ticked number is Case 06099233
Br John Gudmann