Subscriptions: Action bar appears differently based on simple/atomic and domain purchase status

[blackjackkent]

There appear to be a number of different factors influencing how the action bar appears in the bottom right of a site for a user to follow its posts. This may not be an exhaustive list, but:

A simple site with a WPCOM URL displays the follow button and a drop-up with links to managing subscriptions, etc:

A simple site with a purchased domain does not display the follow button, but does display the drop-up control:

An atomic site does not display any of these options:

[kwight]

A simple site with a purchased domain does not display the follow button

That's odd – I have a simple site with custom domain at kwight.blog, and can see the Follow button, can you? Which site did you not have the Follow button?..