(We do not ask for the complete Debug Log, but just for the Debug Token.)
That assumption is correct.
As this is specific to a few domains only, Pi-hole could be involved by blocking access to those domains.
By default, Pi-hole would answer requests for blocked domains with status NOERROR and IP 0.0.0.0. SERVFAIL indicates one of Pi-hole's upstreams returned an error, and you also already explicitly verified (pihole-q) that Pi-hole isn't blocking those domains.
Intermittent SERVFAILs are not uncommon, but you'll hardly ever notice them.
If they persist over a longer period, a DNS server authoritative for that domain may be down, or something is interfering with DNS resolution (outside of your network) - see recent Pi-hole unbound servfail where an ISP was filtering DNS requests (also for further leads on troubleshooting).