Inspired by @yankiara’s recent post:
I spent some more time looking at this - the global styles stylesheet does not seem to be added to the header template in the editor in our case:
Only the global stylesheets stylesheet and global classes stylesheet are added.
I then disabled all other plugins incrementally and when I disabled both WooCommerce and LearnDash, the issue vanished and the stylesheets loaded correctly:
@Louis, it seems like with either or both of these plugins installed, the editor is in a different “mode”, perhaps similar to the iframe vs non-iframe distinction.