When a request is made to a domain that's in the wiki farm, MWHttpRequest automatically routes it to the primary DC. (See the documentation of $wgLocalHTTPProxy and $wgLocalVirtualHosts.) There is no reason to do that for read requests sent from the secondary DC; making the request within the DC is going to be vastly more performant.

To fix it, we'd probably need to duplicate some routing logic from Varnish etc. in the MediaWiki HTTP request layer so it sends to a read or write proxy depending on whether the request is a POST / has a Promise-Non-Write-API-Action header, and offer an override flag and/or hook for special cases.