The world border texture extends too far outwards on the outer corners of the world border. The extended texture is only visible by precisely aligning yourself with a given corner any distance from the outside of the world border.

I have attached a resource pack to turn the world border texture solid white, for ease of visualization/replication (worldbordertexture.zip), as well as a video with the resource pack applied showing the bug in action (worldbordertexture.mp4).

May relate to MC-248304, which impacts world border collision behavior 1 block out from any corner.