USB mount point acts like ordinary empty folder

I use the generic kernel of FreeBSD 13.1-RELEASE-p2. I set up the USB storage device automounting according to section 19.4. of the FreeBSD handbook. Until recently, everything worked normally. However, I now face the following problem.
When I plug in a USB device, a folder /media/da0 is created (as it should). This folder is empty, even though the USB device contains data. When I try to unmount the folder (as root) with # umount /media/da0, the following error occurs:
Code:
umount: /media/da0: not a file system root directory
Independently of letting the USB device plugged in or plug it out, the folder acts like an ordinary folder on the system drive and not like a mount point. That means, I can copy/ move content to the folder, which then takes space on the system drive (hard drive). The folder can be deleted with # rm -rf /media/da0, which frees up the space on the hard drive. When the USB drive is again plugged in, the folder /media/da0 is created again with the described behavior.

My question is if there is a way to "re-assign" /media/da0 as a valid mount point, i.e., recover the usual behavior.
 
Probably not an answer. But i disabled automouting. For the cases i need it i do it manually, so it does what i want, and not what the system thinks what has to be done, which is mostly wrong.
 
Although not being the solution, it was indeed helpful to see that it also didn't work with manual mounting. After testing, I found that the problem is exclusively dependent on the file system format of the USB drive, regardless of its size. That is, when formatted FAT32 it works flawlessly. When formatted exFAT or NTFS, is shows the described behavior.
 
Although technically not the exact solution, the following solved the issue for me sufficiently. The USB drive I used was formatted as NTFS, but apparently was not properly partitioned. After several tests, the exact indicator for a properly partitioned USB drive for my issue is when # gpart show lists the USB drive. To achieve this, I had to re-format the USB drive properly. The behavior with automount remains still the same, however, manually mounting to /mnt/ gives the desired behavior. As this is good enough for me, I consider the problem as solved.
 
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