Another element to this peering/routing issue is that peering can be done on IPv4 or IPv6 or both. For a test. I’ve disabled IPv6 on one of my domains (they are all in the same Cloudflare account, same nameservers).
Domains with IPv6 route locally to a Brazilian colo (GRU)
uag=Mozilla/5.0 (Windows NT …
Domain without IPv6 routes through a US colo (IAD, EWR, MIA etc)
uag=Mozilla/5.0 (Windows NT…
So make sure you have IPv6 compatibility enabled on the Network app.
This setting is ON by default, but it can be turned off via API. This setting can also be turned off if you have a WordPress site, use the Cloudflare Plugin, and apply Optimize Cloudflare for WordPress on the plugin’s Home tab.
In case this setting is Off, there should be a toggle to turn it back on.