I have a lot of experience with that mudguard. If you purchased Puig or other known brand, you can replace the bolt. If you purchased cheap chinese mudguard, do not install that bolt, get a new bolt. There is a big thread here somewhere on this forum about the subject.
However, my experience was: first, the bolt worked its way out so it could be fatal if I haven't seen it just before it felt completely off. Then I purchased longer bolt so I was able to put locknut behind to make sure it doesn't get out again. Well, it has never got out again but after a month, the bracket of the mudguard simply broke in the middle of the ride (I don't go offroad). I heard the clattering noise from the rear of the bike and stopped. Looked at it and one of the two legs of the bracket was sheared completely. Luckily, it was 2 miles from home so I took it easy and then removed the mudguard completely. If it broke completely in the middle of the ride, it could have been fatal if cought by the rear wheel.
So, as you can see, two "almost" ocasions and both could have been be fatal. I have never installed the mudguard again. I have mudsling and it's OK. Well, this is my first BMW and my first bike without mudguard (I'd so much like to have one), after 30 years of owning bikes, but I'll have to live without it.