See
https://answers.microsoft.com/en-us/...e-799ca90d4e58
And macOS too by default is case insensitive (but case preserving).
You simply can not have two case-only variants of a file name in the same folder at the same time on macOS and Windows.
And reusing the same file name more than once using case variants is going to cause trouble on many e-readers as well.
Sticking to fully unique filenames no matter the case is a good design idea for epub developers who want their epub to work with the largest number of e-readers.
The other point is that epub internal file names just do not matter as they are not seen by readers.
So use Regex Rename/Rename to give each file a short, unique, direct file name that either indicates its sequence or function or both.
With RegexRename you can block add an initial char to a number of files in one step. The repeat to remove it and change case.
This works on all platforms.