All full belts stop moving if there is nothing removing content in the Factorio universe. It's obvious to a pre-schooler that full belt that is circular stops moving too.
It's a game. It's not reality. In reality belts need to be powered. Also, in reality, you do not shoot aliens or drive large mechanical spiders. The fact that they chose to implement a game mechanic means it's a game mechanic, not a bug.
The designer didn’t sit down and go “and logically when a circular belt is full, the items should stop moving while the belts keep moving”.
The fact that it was decided to not handle this edgecase does not mean it stops being a bug, no matter how much you love the game or how strongly you want your mantra of “factorial has no bugs” to be true.
Of cause factorial has bugs, lots of them. Just look at the thousands of fixes. This is one of them, and it’s a bug that has been there from the start and will likely never get fixed, but it’s still a bug.
It's a game. It's not reality. In reality belts need to be powered. Also, in reality, you do not shoot aliens or drive large mechanical spiders. The fact that they chose to implement a game mechanic means it's a game mechanic, not a bug.