Every now and then folders get deleted when opening CONTENT\GALLERY in CMSMS
backend

I have a parent Gallery:  {Gallery dir='Vliegers'}
On my website (frontend) I use sub-galleries from this parent Gallery {Gallery
dir='Vliegers/sleeparaplu'}

Below are the steps I take to add sub-galleries:

I used FTP (Lightroom on a iMAC) to upload a folder with pictures into
"/uploads/images/Gallery/Vliegers"
As expected, these pictures then showed up on my website's frontend
(dir='Vliegers/sleeparaplu', is retrieved from a database)
I wanted to have thumbnails, so next step I took was to open the CMSMS backend,
went to CONTENT\GALLERY.
At this point the uploaded pictures didn't show up anymore on my website
anymore, the same counts for some other (recently added subfolders in Gallery)
I then opened the parent gallery in the CMSMS backend, the uploaded folder
doesn't exist either

I have had this problem for several years no, so this was not introduced with
the latest version. I ignored it, by skipping the creation of thumnails, so I
only used FTP (from within Lightroom) to add sub-galleries to my website.