Do you really think that she meant DS is a 'problem child', in the way that you're referencing it? My mum used to say it to me as an affectionate thing sometimes. It really doesn't have the same impact for everybody.
To put it into context, I called my nephew a naughty boy, he's 2, he was on my lap and pulling my hair and I was laughing and calling him it, "Oh you are a naughty little boy, aren't you?". No hostility whatsoever, lots of affection. Anybody who would think I don't love my nephew would be dead wrong, but my brother was horrified... I won't do it again, but truly, nephew and I were laughing and having a whale of a time.
I say it to my own, but in affection only. I say that we 'love naughty boys in our house' and I include the dogs in it! I would never say it if they did something wrong. I would just explain firmly that 'X sort of behaviour is wrong and we don't do that because Y'. When I'm cross, there are no 'labels' whatever. Does that make any sense to you?
Soft grandparents.... what does that mean? Indulgent? I'm sure that your mum loves DS as much as she loves her other GC. However 'soft' a grandparent is, a sensible one will not excuse bad behaviour without saying something just because they don't want to hurt feelings.
Is there something that suggests to you that your mum wants anything other than the very best for you? I think it's very sad that your thought is that she might be jealous... it doesn't make sense. I think you really need to have a conversation and tell her how hurt you are. I bet she'll fall over herself to explain if you give her a chance to do that.
It sounds like you and your mum have lost touch a bit and maybe your visits and calls are on 'tenterhooks' for both of you. Is that possible?
Please don't take it to heart, you sound very sad and I'm convinced that your mum would be horrified that she'd upset you like this.