Once you teach a dog to bite no matter how well trained you can not trust them.
They live for the bite like any trained dog they love their work.
I have a mali. You don’t have to teach them to bite, you have to teach them to out, for the most part, you give them an opportunity, they will bite.
You never know what may have really happened. The first ‘protective’ type dog I owned was a really good dobe 30 years ago. One day I heard him barking/growling and a teenager was walking by, when the dog came to the fence he was smacking a baseball bat against the fence right in the dog’s face. Common sense doesn’t rule. It sounds like there was a malfunction, and a malinois has tremendous prey drive, if the guy took off at a run, he gave chase, took the proper bite on the upper arm, and held on.
They love to bite so much, that you use the bite as a reward in training. They are kind of odd, they will pretty much bite with a ‘smile’. In other words, don’t tend to be vicious, often don’t growl, it is all a game/play to them. It does make them easier to train/handle because normally they aren’t in a defensive posture, they are just playing.