The bug

Water placed within the bounds of the world border are capable of flowing out of it.

This was originally reported as MC-54772, which was resolved as a duplicate of MC-54119, which was fixed in 1.18 Pre-release 1, but this specific behaviour has still no been fixed.

How to reproduce

1. Go to the world border
2. Place a liquid source block which would flow outside of the world border

Expected results

As terrain outside of the world border is meant to be protected, the water/lava would not flow outside of the world border, either stopping or treating it as a solid block.

Actual results

It flows outside of the world border anyway.