I was having this issue since 25.5 and now I'm at 26.1. I've had to keep restarting OPNSense just so I can update to 26.1.10. I've increased the the tunable "kern.ipc.maxsockbuf" to 10MB as I've seen in a previous post before.
I don't have any VPNs configured, yet. I have a separate internal DNS server that I use for internal network. I only use Unbound for OPNSense. I've tried pointing system to my Internal DNS server and that works, but I really want to use Unbound only for OPNSense, but I don't know what else could be wrong.