The Issue
In 1.17.1 and prior, running through the corners of a world border while outside of it would allow you to pass into it without issue. As of the latest affected version, you get stuck on the outer corners of the world border while trying to run through them, as well as a block beyond the corners in either horizontal axis. This corner collision behavior feels unintuitive and inconsistent with that of the sides of the world border, which are only solid when you are inside it.
Steps to Reproduce
Establish center of world border:
/worldborder center ~ ~
Set world border width to something manageable:
/worldborder set 3
Fly (via spectator mode) or teleport outside of the world border
Observe collision behavior when walking into/around corners of world border from the outside
Note
I am reporting this because it impacts a minigame I made about running into the world border while outside of it. The first video attached demonstrates how the world border previously behaved in this minigame as of 1.17.1, and the second video shows how the world border behaves as of 1.18.1 and all future affected versions (which I believe to be broken).
Appears to happen when you increase/decrease the worldborder size.
Easiest way to replicate:
1) /worldborder set 10
You'll notice that the worldborder behaves as expected.
2) /worldborder set 10.0001 100000
(or in case of decreasing size: set it to 9.9999 100000)
The corners of the worldborder now get this unintended solid effect.