When you delete the first character in a line containing a breakpoint, the breakpoint is deleted.

[Claus Pedersen]

I do not know if this is by design, but when you delete the first character (position 1) in a source line, the breakpoint on that line is deleted. If you delete a character at position 2 or 3 or ..., the breakpoint is not deleted.
It does not matter whether the first character is deleted by pressing Del or Backspace.

Bookmark on the line is also deleted.

Is this intentional?

Also an issue in version 12 beta.

 

[Marco Kalter]

It is not by design, but this is indeed a consequence of how the bookmark system works internally.