iPad ePub using ISBN as filename only shows blank pages

[Cepheus]

Here's an odd one.
I'm using an iPad (9nth gen) running the 18.5 OS and all software is up to date.

I uploaded an ePub to the iPad by saving the file from DropBox. The ePub has the filename '9780829460001.epub'. When I open the file in iBooks there is no content. It is all just blank pages.

Here's the odd part. If I add a letter to the beginning of the filename the ePub works correctly (I changed it to 'T9780829460001.epub' and newepub.epub)!

Does anyone know why this is? Many of our vendors want the filename to just be the ISBN number.

Thank you for any help