Uhm... but your example box does not follow the rules of perspective, it's an object (that is indeed not a box) in what is close to isometric view. there are no shared vanishing points, and your lines are not converging aside from what I think are accidental inaccuracies. Or if they're not accidental, then they're using them incorrectly. The two lines on the z-axis of the front plane for example converge slightly towards the top, but we're looking at the object from above, so the vanishing point must actually be below the object.

The "rules of perspective" you're talking about is not actually a "rule of perspective", it's just what happens when you draw boxes in perspective while following the actual rules of perspective. Any object that has far plains that are bigger than the near plains cannot not a box. But it can still be an object that is drawn correctly in perspective. And just because an object has smaller far plains than it has front plains does not automatically make it a box.

Drawing a box just means that all lines of the three axes must be parallel in 3d space, which means they must converge towards shared vanishing points in 2d space (this is of course assuming 3-point perspective is used for the box).

I'm a bit too tired to think about whether the rules you gave are actually correct, but in either case, I don't think these rules really matter all that much, and I think you're going at it the "wrong" (and by that I indirect) way. If you're using vanishing points the way Uncomfortable introduces them in the lesson, then you should not have to think about any of this. If you follow the rule that all lines that are parallel in 3d space have to converge towards a shared vanishing point in 2d space and start with a line for each axis, then a box will always turn out correctly no matter what you do. The box may end up looking very dramatic, but it will still read as a box.

So if you end up with a lot of boxes that are not actual boxes, then I think you should probably re-read the lesson, because you're probably making constructional mistakes.