Today I accidentally bought raid energy when I already had 5 flags. (It was a completely accidental purchase, but not relevant.)
The game already disallows buying raid energy when you are full. Since nothing in the game takes more than 1 raid energy it would be a quality of life update to prevent purchasing more when you still have some.
Hopefully, not many people are doing this, but it’d be great if the game didn’t allow it either.