So I mean since you have no DNS issues detected in ha resolution info you can try disabling the fallback DNS by doing ha dns options --fallback=false. That shouldn’t fix it but if it does please submit a bug here. It thought I had fixed the issue where coreDNS would get “stuck” on the fallback but perhaps not.
Although I have to say the real headscratcher is that nslookup adguard.lan 192.168.1.2 didn’t work. You’re certain if you run that same command on some other machine on your network it works? I mean that command basically doesn’t even involve HAOS. The SSH addon is essentially just an alpine linux system and its talking straight to port 53 on your machine via nslookup. Which I assume Adguard is listening on. I’m really at a loss how that could fail but some other machine on your network could succeed…