New branch button not showing

This is related to the feature that was introduced with !2808 (merged).

In one of my projects that was forked from another project, I cannot see the button that lets me create a new branch (community edition). It shows for a second and is then hidden as the page loads. In the parent project that I forked from, I can see the button though.

Parent:

Fork:

For a fraction of a second I can see a grey button that says "[spinner] Checking branch availability..." at the place that the button is supposed to show.