This was exactly my problem. As long as the pictures / files are on the "system"-hard drive everything is fine. The behaviour just starts when I took a picture from a another disk (for example network drive, SD-card, second internal drive, external usb drive).
Thats very anoying when you have to play arround with large amount of files and had to copy it first to the first hard drive.
Does anyone know why iMessage don't send from other drives?
Has it something to do with file system?
Has it something to do with right management?
Wasn't able to do further tests.
But can tell, that my second internal drive is APFS (Encrypted) formated, like my system hard drive - so I would largely rule that out.