There is something strange with your folder panel, and I wonder if that could cause this problem. Your folder panel shows two drives, but these drives should have a drive letter or a name. They don't have any drive letter or a name, however. That means that Lightroom has no way of knowing the difference, because Lightroom uses a simple path to the folders on the drive.
Thx Johan for your reply .
This is where LR is putting the missing folders, but with references found in the catalog. I have more from a previous import!
I'll remove the extra occurrences. It's still interesting to see the dupes, even before I mapped the right folders, f.e. 2018-10-23_Helicopter was duped already... which looks strange to me.
There must be something with the NAS. I only use local disks myself, but as far as I know, a NAS should appear in the same way as a local disk, so with at least a drive letter. The fact that all these missing folders have a zero image count also shows that Lightroom is quite confused.