"I've joined a Pixelfed instance with my Mastadon mstdn.ca login and that seems to propagate"
I know Pixelfed advertises it as using a login on both servers, but that isn't really what is happening.
All that Pixelfed has done is created you a new account on the pixelfed instance, and copied over some of your profile info from your Mastodon account. But the two accounts are separate.
Accounts aren't linked on the Fediverse, they are always tied to specific servers.