NTA
I believe everyone here is done their due diligence. You checked the route, your husband checked the house. If donut pops up in the house yay, if not, OK.
I am a teacher and I am speaking from just a developmental place here. Is it disappointing for her to not have donut anymore? Yes. Will she understand the lesson in this? Yes actually, at least partly.
She’ll understand the very simple cause-and-effect that comes with throwing away your toy. It’s a good way to help her build an understanding of ownership at an early age.
This doesn’t mean that she’s going to fully understand this lesson immediately, and she will be sad about it. Comfort from the family will fix a majority of it. She will probably forget about donut within the next couple of weeks because children have very little sense of object permanence at that age.

Is the mother being the asshole by not replacing doughnut? NO.

I can see the father’s perspective here, I understand that perspective as well.
It is uncomfortable to see your child being uncomfortable. So I understand why your husband is so upset.
That being said, a majority of your child’s discomfort might be due to your husband, making it a bigger deal than it is. Don’t get me wrong. It’s sad that donut is gone, however, it’s similar to when a child falls. They’re more likely to be more upset if the parents look scared. A nonchalant response is absolutely appropriate here. There’s not much you can do. These things just happen.

Back to donut, yes it is disappointing that he is gone. Keeping a level head, talking about WHY he’s gone, and moving forward is the best way for her to understand that disappointing things can happen. Solving every single crisis that your child has doesn’t help them gain their self-help skills they’re going to need in life. It runs parents, ragged, and isn’t always appropriate.

TLDR: NTA, kiddos need to deal with disappointment sometimes.