I appreciate the replies and will examine the configuration to see if there are differences. I've run so many queries that 2 months ago when I started the draft I was seeing results like ;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 36534. I included more recent results when I decided to try posting here. Incidentally the device was setup on 29 April and didn't experience issues until 13 May. It took me so long to detect it that I don't know what updates if any might have happened during those 2 weeks to influence the issue.
It seems unlikely my ISP is blocking that domain because queries using 8.8.8.8 go through right away. I admit I don't understand what recursive DNS really means, but it seems if the ISP were blocking the site or if the site had its own issues, then it wouldn't work with 8.8.8.8 either. My ignorance aside, I'll keep trying to find a solution. There are dozens of posts complaining about this specific domain not working with unbound + pi-hole, and none so far seem to have a successful resolution.
I was reading another post yesterday where certain .il domains were failing to connect in the same fashion. It was recommended there to install systemd-resolved, and while that seemed to help the poster in that thread, all it did was break pi-hole for me, causing FTL to fail to load because port 53 was already in use. I uninstalled systemd-resolved, and the system went back to working.
Anyway thank you again for the assistance. I'll keep looking around and maybe ping DietPi support again to see if they have ideas there.