Time Machine defaults to backup to my NAS and not to my external time machine formatted drive

I have an external drive which is Time Machine formatted (APFS Volume • APFS (Case-sensitive, Encrypted)), and I also have on my NAS (on EXT4 formatted) created a share for Time Machine.

Now what is strange is that macOS prefers to Backup to the NAS (schedule set to Daily) and not to to the external drive.

Of course I can do manually backups to the External drive but I find it strange that Time Machine defaults to backup to the NAS and not to the external

I have of course tried a backup first to the external and 1 hour later to the NAS, and then waited a day or so. And what I found is that Time Machine had backuped to the NAS and NOT to the external

Not a major issue. but just wondering as I am reading everywhere that Time Machine prefers to backup to APFS Volume • APFS (Case-sensitive, Encrypted) drive.

Edit: The External Drive (usb connected) has the TimeMachine Logo when connected and the NAS doesn't