Apple books refusing to open epub

I need to find a way to resolve an issue that has been ongoing for a couple of months now and seems to be lost in techno-weeds. I’m wondering if anyone else has heard about this and knows anything about a cure.

I am a designer/developer of epubs and have been working with a self-publishing author of a FXL comic book (epub format). The book was completed and tested with epubcheck and imported into various readers (IOS and Desktop Apple books, Thorium, GooglePlay) before being passed to the author who then passed it on to the self-publishing aggregator which supported the format.

The epub has been distributed to Kobo (where it is readable) and to Apple. At the Apple store, the message a purchaser gets, either for sampling or upon downloading is:

'Cannot open [TITLE of EPUB] It is formatted incorrectly, or is not a format that Apple Books can open.’

This issue has remained un-resolved for over 2 months, despite many hours spent in consultation, alerting respective technicians to the problem.

Does anyone know/have heard about this problem with Apple?