[Bug?] Can't follow certain users on Mastodon instances

I've been encountering a bug when I try to follow certain users from mastodon instances. I'll click the follow button, but get no response. Following other users on the same instance works fine.

I'm curious to figure out if this is a Kbin bug, or if there's a user account setting in Mastodon that they have in place that defederates the account and prevents it from being followed.

I'm also curious if our "Federation Status" switch in our profile prevents us from being followed in the same way (would love more detail on this function).

Tagging @ernest in case this is a bug. Thanks in advance for any insight.