This issue is specific to the characters related to normalization in the Unicode set which can be used interchangeably. These characters are handled differently by Mac and Windows.
Windows treats the URL as a black box and creates files with names provided to it. Therefore, files having file names differing by a single character can co-exist. E.g. 郎.indd and 郎.indd can co-exist in the same folder. On Mac, the behavior is a bit different.
Till HFS+ it was recommended to normalize the file before saving it, although there was no OS validation and you can save both the files. This allowed files having different file names before normalization but same file name after normalization to co-exist. For example
However, with APFS it is now necessary to normalize the file before saving it. Therefore, files having different file names before normalization but same file name after normalization cannot co-exist. For example
There are scenarios with shared location workflows where the user is on a Windows machine but the shared location is present on a Mac machine. In these cross-platform scenarios, InDesign should be able to save or access files. However, because of the difference in the handling of special characters between Windows and Mac, InDesign is unable to save or access these files across platforms.