Without looking at it it’s probably making a unique request to a resource on a NextDNS subdomain and watching where the request comes from. Like pulling an image from (unique _string).check.nextdns.com. This requires nothing special on the client, it’s making a standard request, and as part of that it needs to do a DNS lookup.
If the source of the and your IP are similar then it’s likely the same network, otherwise it can correlate the source with known resolvers.
This sounds like you’ll need to do a balance operation. Try this first and see if it helps:
btrfs balance start -dusage=0 -musage=0 /If not you can increase the number to 5 or 10. This operation reallocates chunks on the disk and ensures they’re filled - check https://btrfs.readthedocs.io/en/latest/Balance.html for details.