Especially while processing the inbox this feels like a real annoying bug.
Let’s say I want to process an item as next action with focus star, I’d have to set the focus star first, otherwise, after pressing save, the item will be out of sight and I’d have to look for the item in my next action list to set the focus- star.
I’m assuming this is about desktop. Yes, this can be annoying, but it’s clearly a missing feature and not a bug :).
I believe the editor view should definitely have a focus button, as indicated by the fact that both the Android and the iOS app have those.
In the meantime, the tip by @raullg is the way to go. It may even be easier than having a focus button.