@scarleton - thanks for this, I hadn’t realized there were custom settings for this (I was looking on the WAN page, not on the DNS Proxy page).
FYI, there is definitely some sort of bug using VLANs with DNS as described here: Outbound Policy + VLAN bug in 8.5 and 8.4.1 so if your setup stops working abruptly, this may be why. The bug seems to require one to be using an Outbound Policy on the VLAN.
Are you using Outbound Policy?