Wireless PRO and iOS Files app limitations?

I have an interesting problem while using the files app with the WD PRO (extFAT).
When I have the drive direct connect to iOS device, browsing certain folders, not all of them, using iOS 13 files app I am getting the iOS message “Content Unavailable”.
When browsing the same folders in the WD app I can do so with no error. Of course browsing on PC works too.
Any idea why it’s different? Doesn’t seem to matter the what level the folder is in or the naming.
The largest number of files in any one folder is 78.
The largest file in any one folder is about 4GB.
Thanks in advance.