files wrongly marked as executable - Cinnamon/ Nemo

[tpprynn]

I think I remember encountering this before, whether it was when I used Mint or during an earlier try of Cinnamon on Debian, but I have just reinstalled Debian 10 Cinnamon edition and moved files from the previous install from a USB drive. They all have the tick in the box showing they're marked as executable - pdf files, text files, Libre Office files, jpg and png files. Files in /home generated by the install don't. The external drive is NTFS-formatted if that's relevant, but I hadn't seen this sort of thing when using Xfce or LXDE.

How do I undo this, and what have I done 'wrong' that caused this? I have tried reading around the topic and see reference to the NTFS drives possibly being involved, but it's not unusual for us to have external hard drives formatted this way I would think. It was quite a long operation recently for me to move the contents of a computer's data drive onto three smaller external drives to be used with laptops so I'm not too keen to redo this with FAT if that would have been simpler.

Is it a Cinnamon/ Nemo quirk?

Thanks.

If relevant, here is /etc/fstab:

# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point> <type> <options> <dump> <pass>
# / was on /dev/mmcblk0p2 during installation
UUID=5a062430-9ecf-421a-9f1e-8617bcdaab97 / ext4 errors=remount-ro 0 1
# /boot/efi was on /dev/mmcblk0p1 during installation
UUID=9206-E29C /boot/efi vfat umask=0077 0 1
# swap was on /dev/mmcblk0p3 during installation
UUID=12e2d5ac-73c5-471b-8ddb-1283d39285ac none swap sw 0 0

[p.H]

It depends how the NTFS filesystem was mounted before restoring the files. NTFS does not support Unix permissions, so by default all files appear to have all permissions (rwx) set for everyone.