Mobygames grants contribution points for a wide bunch of elements, which is quite nice. You recive points for contributing and lost points if your approved contribution is replaced. Everything is send to the user with the contribution messages. So far so good.

But one thing is strange: I lose contribution points without any message that something was replaced or removed. At last I lost 16 points between Thursday and Saturday. Is there a bug in the contribution messages or is the database loosing entries?

You might have lost points due to corrections. For example if a company is removed from release info you'll lose one point.

Should't I get a contribution message here too?

Not a notice that shows up on your front page, no. It should (I think?) show up under rejected if you check your approval information page.

No, because for handling corrections information is manipulated directly, not through the contribution system.