According to the company for which I work, an endless spinner is for sure a bug. Let us let iNaturalist say if it is a bug according to their standards, I won’t discuss it further.
There are many cases in web development where the URL does not change when filters are applied within the page. I would prefer on iNat that a change in the filters is always related to a change in the URL, but I can’t say that it has to be like that.