No, that's not it at all.
I use the "add" feature during import, which has always added folders to the catalog and maintained the correct folder hierarchy.
And then, it suddenly stopped doing that, and, when I try to "show parent" for the folders that are appearing at the top level of the folders, I get an error that the parent value is null. So, the problem is Lightroom.
If I create a new catalog, and add folders to it, the problem goes away. That certainly fixes it, and I've tried all the options to fix the existing catalog, but, the fact that it happened with every single catalog after a release in 2015 says to me that it's a Lightroom bug.
Well since I can't see your computer, I am not convinced that it isn't the same problem. Uusing "Add" doesn't convince me it's a different problem. Folders in the Lightroom Folder panel that appear to be out of the right hierarchy are almost always this problem. So I still suggest you at least try the solution there. But it's up to you.
Yes, I have tried that solution.
That solution is regarding folder names being imported to a folder whose name is capitalized. The add function doesn't move folders. It simply reads files on disk in a specified location and then adds them to the catalog. The folder then shows up under its parent folder.
Not sure how to make that any clearer. Sorry.
It's a process I've followed since day one of using Lightroom, and the process works if I create a new catalog.
Again, not sure how to explain this any more clearly.
Could you please post a screen shot of the folder area.
Could you create a new folder under one of your other folders using your File Manager and copy a few new images into it then import it into LR using the Add option in the import dialog window. Then post a screen shot showing that new folder.