This is a feature of Medium Editor, the engine behind the editor field and controls. If text is not selected the buttons are disabled. I believe that having the buttons always appear would be confusing to users as to why they don’t do anything. I have to choose between one annoyance or the other and since one of them would cause more confusion it’s the one I chose.
The other choice I have is the there will always be a gap equal to the button bar between the label and the editor field. I thought that this looked bad.
If I get enough people tell me they’d like to see it work another way then I’ll consider it. I put up an issue about this over on github here https://github.com/Hube2/acf-medium-editor/issues/12, if you have a preference then post what it is over there.