Oh, then the ntfs-3g package is not installed? which would be of course a good reason:
apt install ntfs-3g
An ntfs kernel package, without trailing 3, that is interesting. Never saw that one, Paragon’s is called ntfs3. I’ll check on my XU4.
EDIT: Ah, ntfs without 3 is the classic read-only kernel driver for NTFS. Now it makes sense. Installing the above package should fix it. Just strange that it was not installed automatically by dietpi-drive_manager 
.