Despite changing the default VLAN to 2, this hasn't solved this problem as I just got another alert. I have now raised a case with Meraki support, who were equally as stumped. Because the Unifi switch is blocking the DHCP request, the MX isn't reporting anything helpful in the Event Log. A case has been created and I will watch for the next alert to try to establish activity around that time (clients arriving, joining the network, leaving, new devices plugged in, etc).
I have also turned on remote syslogging for the Unifi switch, so this will hopefully help identify what is happening. I'll update this topic when I have more to share.
Strange. Maybe the MX is using the 23.1 address of the highest VLAN for the DHCP server identification for the DHCP Offers sent to the 1 and 10 VLAN and is that what the Unifi switch is telling you instead of the actual L3 source address it saw on the packets.
If I were you I'd take a packet capture on your unifi switch connected to MX port 1. I would want to see the details of that DHCP request.
Make sure the DHCP is turned off on the unifi switch. Also best practice would be to change your 192.168.1.0 to 192.168.10.0. reason being is that alot of home wifi or public wifi is in that subnet. We have seen the issue with Macbooks more on the VPN, but they will resolve 192.168.1.0 to a local IP rather than the VPN subnet, if the MX default is 192.168.1.0.
@SoCalRacer Thanks. DHCP is turned off on the Unifi Switches. The IP address is not exactly 192.168.1.0 as I described - I just simplified it to that for my explanation, but thanks for the reminder about using such a common subnet.