I've followed Ole's coaching career since Molde.
He's a weird fella. It's strange how poorly understood he is (though of course with time that changes) in England.
Let's just say he's very far from being "a saint", this good guy who always does the right thing. He's a wolf in sheep's clothing. And he holds a grudge. He boycotts norwegian media for years simply for referencing legitimate questions about the many dodgy things he's been involved with. The media that actually reports on it stands no chance ever of getting anything from him.
So back to the Donny situation. Ole is stubborn and not very flexible and in some cases not very reasonable (see f.ex. the Sarr situation).
He clearly doesn't rate Donny. The reasonable thing would be to let him go and recoup the losses. However that means losing an option in midfield. Rest assured that Ole doesn't want to lose options unless he absolutely needs to. Hence Donny stays. And this is not so strange.
The next reasonable event in this sequence would be that Donny gets to play more, at least in low risk situations. Yet, that doesn't happen either, because again Ole is not very flexible.
If Donny was a scout signing, it could factor in the way he's being treated. It's a bit speculative yes, but Ole holds a grudge. If he's had disagreements with the scouts, which is not far fetched to assume, or is still bitter over his Haaland recommendation that wasn't picked up, then that could factor in. Is he that much of an asshole? I don't know, but it's hard to find explanations for why such an expensive signing has been given so little time. Usually it's not even a question about it, even if the player is truly shitty, he's given time to prove himself when he costs 40 million.
Even if that is the case, it's not that Ole won't use him ever out of principle, but he won't get anything for free either. And I don't think it's personal - I'm sure he's trying to build up Donny on the training ground, he's just used as a pawn, to hammer in the message that Ole knows best. Donny will be used when he's truly needed and that's it.
Anyway, speculation and all, but this is my best guess of the situation.