@bruce Same issue here on Flint with OpenVPN + Adguard since I upgraded to 4.6.4 and also 4.6.6:
It wont resolve the OpenVPN server name if Adguard is enabled and you have enabled "block non vpn traffic". it worked before 4.6.4 fine. it also works fine on my Brume 2 with 4.6.6 with same settings, except I use WG there not OpenVPN.
You need to exclude Adguard from the "block non vpn traffic", it is a service of the router itself and also the main DNS resolver if you use it.
I have not changed anything on Adguard except add more servers:
tls://unfiltered.adguard-dns.com
tls://1.1.1.1:853
tls://1.0.0.1:853
tls://8.8.8.8
tls://8.8.4.4