Did you consider training him to do it "right" ?
Why do you think he should bring it back to you in the first place?
I don't know why he does this and I can't find any reason either. But that's not the point.
Bringing something back can be more or less "natural" to some dogs, but in any case dropping on cue at the handler's feet needs to be trained.
It seems that you have some reasons to be very positive:
- he's already bringing the ball back to you when you're alone
- he's dropping it at others people feet (I doubt he does that 100% of the time, but let's assume he does)
So that's your starting point and it could be much worse...
So now it your job to teach/train him what you want.
As alway, focus on the behaviours you want and then teach them.
I'm a bit worried when you say "very obedient and passive", that doesn't really sounds fun... I strongly suggest that you have a look at training methods that are labelled as "positive reinforcement" or "clicker training" or any variations of these four words.
Have fun !