DHCP clients have no Internet after DHCP network change

[verfluchten]

es, if the 192.168.12.0/x subnet overlaps your LAN network, its route may take precedence. You cannot have two overlapping networks on the same router, because it it unclear where packets should be headed in that case.

I do not follow your logic. My LAN iface network is 192.168.0.0/16 which allows for any address in the range of 192.168.0.1-192.168.254.254, so I am free to use any subnets be they .10.0, .11.0, .12.0 or .254.0. All of the subnets with static IPs work fine but not the DHCP subnet. What am I missing?

You cannot have two overlapping networks on the same router,

Unfortunately, I do not understand your terminology. What are you referring to as "two overlapping networks"? There is only one network assigned to the LAN iface 192.168.10.1/16. Which is the other one?

[verfluchten]

Post the output of "netstat -rn" on your OPNsense, please. You can obfuscate your WAN address but we need all the internal RFC 1918 networks. They are not privacy relevant/critical.

I think you nailed it! There is a duplicate 192.168.11.0 network that overlaps with my DHCP range. I totally forgot about the now-unused OpenVPN which uses it. Thank you so much for this simple diagnostics that I forgot to run!