This has happened to two books I purchased and have recently tried to read on my Aura 2. Both are readable on ipad and show as readable on ereaders in the icon list in the kobo store.
The only thing odd about one of them is that it shows a blank cover in the kobo app. The other looks fine.
I can sideload via Calibre, but I'd rather not

.
Anyone had this happen? I've had kobos since the original Touch and it's never happened with a purchased book that indicates it's readable on ereaders. It even tells me I don't have to download the book and can go directly through the ereader when I press the download buttons in the new library format.