I know that there is a formula. But there is no visual clue that lock/unlock of the button is managed by a formula. (feature request to add a visual clue).
G23 counts the number of Fuel Log records created by clicking the button and attached to that Vehicle. But the counter in G23 is not directley updated. The conditional lock does moet fire
What you are describing is that you are adding a child item, but the count of children is not updating in G23 and as a result not enforcing the lock.
It is possible that the record is updated but the UI is not showing refreshed record.
Can you try clicking the refresh icon on the top right corner to see if it works correctly after you add a child without having to leave the record and come back again?