tldr; IP6 lookups seem to be defaulting to ISP even with a custom DNS servers configured. IP4 seems to be working correctly.
I know a lot of folks have opinions on NextDNS and using it with Firewalla, but there are other reasons I use it and need to keep doing so. I have it configured through MCP with the DNS-over-HTTPS url and applied to all device. This works for devices on my network that are forced to IP4 only (via MDM for kids devices) Note: nearly all Apple devices.
Recently my own devices have been resolving via COMCAST. When looking at my device DNS I see my Firewalla's IP, but also a Comcast IP6 address that seems to be coming from Firewalla. This has been going on for a few weeks and I'm stumped on how to stop this from happening.
Source: r/firewalla · by /u/PeltedVenom