Steps to reproduce:

Create new post with:
https://example.com/test/#?javascript:lolhax

What happens:
https://example.com/test/#?javascript:lolhax

What should happen:
https://example.com/test/#?javascript:lolhax

You can also use the preview option here https://www.phpbb.com/community/posting.php?mode=post&f=26#preview to test it.