In the last level it shows a countdown starting 9 which says "dead in" when I am hit by something.

Normally I should be directly dead.

Mh, seems this is not a bug but a feature, this death counter.

Well, for me it fells like a bug, because why can I reset the counter by killing someone and why does me a mine not directly die).

I think it is a feature that is there simply to allow you to get some foreknowledge of the level, kind of like a "hey these levels are a bit tougher so we'll let you tinker with how they operate a bit instead of outright kill you immediately"