Pi-hole would use ns1.pi-hole-net for retrieving its supported OS versions, which isn't 209.107.219.5.
I fail to see how UDP buffer size could actually be involved in this, as retrieving the versions involves DNS replies of under 400 bytes, which is well below the 1232 byte limit.
You'll see MSG SIZE as part of dig output if you omit +short.
You should also be aware that pihole-FTL/dnsmasq has been using edns-packet-max=1232 by default since Pi-hole FTL v5.22 released, i.e. as early as March 2023.
You should consider to approach StrongDNS support with your observation - it's a paid service, after all.