Books disappering from Kindle

Hello everyone,

Since today I have a weird issue with my Kindle Paperwhite.

I tried to transfer some sideloaded *.AZW3 books to my kindle (via calibre).

After disconnecting the kindle everything seems fine and the books appeared in the kindle library and I can read them.

However, when I connect the kindle to calibre again the books just disappear from the library.

Any help would be appreciated. I am using a Kindle PW 4th generation. My Calibre version is 5.7.2