I don't know enough English to explain it properly.
It is a minor visual bug that came with the auto-level up feature. The first time you click the button, you have leveled up but the page doesn't update on your side, you are still seeing the page before the level up. If you change to another page eg. upgrade, you will see you have indeed leveled up.
Clicking the button a second time when you can not level up does nothing but update/refresh the page. So it looks as if you would need to click it twice.