The bug
Player can walk through worldborder when set to less than 3 if they were put outside of the border once the size was changed. For larger border sizes, once they walk back inside of the border, they will be stuck inside of it, but at sizes less than 3, they do not get stuck.
How to reproduce
/worldborder center ~ ~/worldborder set 10Go to one edge of the worldborder
/worldborder set 2
→ You can walk through the worldborder
Note
When you reopen the world while standing inside the playable area the worldborder works again
Comments 8
Looks like your second comment and attached picture were intended to be added to a different ticket =)
Confirmed.