I am not aware of any loot boxes that aren't random? Are you just saying this is possible, or do you know of an instance of this?
I have vague memories of reading about this kind of dark pattern when I first heard about loot boxes, but I failed to google any references now so maybe my memories are diluted with imagination.
But why wouldn't it happen? How would the public find out? Tipping the scales just a little bit, at the right times, for the right victims, could be very effective at "increasing engagement". The whole thing would even be a/b-testable, so any internal arguments against would be futile. (You'd want to keep the "a" implementation around anyways, in case you need to present source to someone who should not see how the sausage is made)
I'd expect that's part of the point of regulating something that appears to be gambling as gambling, to ensure that it is, at least, fair (for some definition of that word).