ok, after a deep dive I think I got what's happening here (haven't used mongoose in almost a year! so I had to refresh)

First: getQuery() is deprecated and the new equivalent is getFilter(). mongoose docs say:

You should use getFilter() instead of getQuery() where possible. getQuery() will likely be deprecated in a future release.

Second: you want to get the updated document to calculate the review right? then what you have to do is only accepting the updated document in the post hook since it's one of the accepted parameters:

reviewSchema.post(/^findOneAnd/, async function(updatedReviewdDoc){
    await updatedReviewdDoc.constructor.calculateAvgRatings(updatedReviewdDoc.tour);
});

your post hook didn't work because you must have thought that the this keyword is shared or inherited between both the pre and the post right? but that is not the case.

let's break it down, you're using one of the Query middleware/hook so this keyword is referring to the query itself not to the document, that's why your post is not working as you might think it would. you can't call constructor.calculateAvgRatings on a query instance! constructor property belongs to a Mongoose document.

yes you fetched the document here:

reviewSchema.pre(/^findOneAnd/, async function (next) {
    this.r = await this.model.findOne(this.getQuery());
    next();
});

but as I mentioned this is not shared or inherited.

Finally, you can safely remove the pre hook you don't need it since the post can get the updated doc without any help from it.

I hope my answer makes all the sense and helped you