How about we meet in the middle...
You did acknowledge just making a change in version 1.555 to, and I quote "The Neighborly Help screen should not show players outside your scouted area.". What does this mean? What process, running how often, under what condition, was modified to effect this change?
Is it possible that that change removed or modified some little subroutine call that previously was being performed 350 times after every NH was given, and adding a 25ms delay for every already-helped neighbor? Maybe the test no longer takes place. Maybe the test changed what information it looks at? Maybe the data it accesses is now locally cached and so no longer suffers increasing penalties as it was being increasingly accessed?
So, maybe you have intentionally "...made no changes at all to this...", but nonetheless have ended up making changes that did.
Heck, I'd be willing to personally roll back to v1.554 and spend 10 minutes or so seeing if that made a difference.