When outside a world border, you realize there is a second world border that's invisible, when you try and throw items throw that border the items surprisingly go through the border even though they're not supposed to since its a border. Now try this with the first world border and you will notice that items cannot go through that border and start glitching.

How to reproduce

Start by teleporting outside the world border using this command:

/tp 29999999 256 29999999

Then grab any item and throw the item at the invisible world border, you will notice that it can go through the border.

Now repeat these steps on the first world border, you will notice that the inside world border prevents items from going through the border.

Observations: Invisible world border allows items to go through, but the world border doesn't allow items to cross.

Expectations: Since the world border doesn't allow dropped items to go through, same thing has to go for the invisible border located outside the world border.