When a purchase link breaks, it almost never means anything weird or dramatic happened. Most of the time, it comes down to exclusivity shifts behind the scenes.
Authors move in and out of programs like Kindle Unlimited and Kobo Plus all the time. Those programs usually require temporary exclusivity. When an author switches, retailers quietly remove or disable the book listing on the other stores. The old links still exist, but they no longer lead anywhere useful.
So a Kobo link can stop working when a book enters Kindle Unlimited. An Amazon link can stop working when a book enters Kobo Plus. Sometimes a book gets re-uploaded as a new edition, which also breaks the old retailer URL.
I try to keep everything current, but these changes happen without notice. Your heads up helps me fix it quickly so other readers do not run into the same dead end.