Its a fixed layout and I use the Digital Editions reader delivered by Adobe itself. Validating isnt an option, because the file has way more GB than allowed to upload.
I think, there is not much you can do, it's the reader app Adobe Digital Editions you are trying.
You may have better results with Thorium Reader.
( ACP )
I´ll try thorium
Thorium Reader is far superior to Adobe Digital Editions. Quite frankly, Adobe should be embarrassed at this point.
FXL EPUB is not new anymore and the fact that they can't even get their own app to read the output from InDesign is inexcusable.
The Windows 10 version is available here: Get Thorium Reader - Microsoft Store
Thanks for the input Bob, will do!