Trust Levels.
Most forum posters are TL2. TL2 posters are restricted from posting live links to most websites. (There are exceptions; for instance, TL2 could post links to other forum topics or posts, the last I checked.)
Trust Level 3 posters have that restriction listed, although I’m sure there are still some safeguards to prevent linking to genuinely inappropriate content. TL3 requires significant participation in the forums over a 100-day period.
If you want to include a hyperlink but don’t have sufficient Trust Level to post it, you can insert it as a dead text hyperlink by surrounding it with backtick characters:
`https://www.google.com`
which looks like
https://www.google.com/
It isn’t a live link, but it’s easy to copy/paste and the system will accept it.
And no, it’s not a bug. It’s a deliberate implementation of the website development team. I don’t understand the reasons, but that’s not really required, is it?