You'd likely be unable to resolve any domain if your anchor was incorrect.
Assuming that you are just seeing those SERVFAILs for certain domains, your issue seems to be upstream.
SERVFAIL indicates one of Pi-hole's and in turn unbound's upstreams returned an error.
Intermittent SERVFAILs are not uncommon, and you'll hardly ever notice them.
Unfortunately, they are somewhat hard to troubleshoot if they persist over a longer period.
They may indicate a DNS server authoritative for that domain may be down, or something is interfering with DNS resolution (outside of your network) - see Pi-hole unbound servfail where an ISP was filtering DNS requests.
(Some) domains returning "SERVFAIL" but no DNSSEC enabled - #9 by deHakkelaar also may have some leads on troubleshooting issues with resolving mail.protonmail.com.