Side loaded books not appearing in Kindle library

[Brissieboy]

I have been using Calibre (currently latest at 8.7) on a Windows 10 PC with my Paperwhite 7th for many years with great success. I do use deDRM and have always used mobi for conversion.
My new issue:
Now when I 'Send to device' Calibre recognises that the book is on the Kindle, but it does not appear in the Kindle library.
I have tried different conversion formats (mobi, kfx) with the same result. There are no problems with the conversion from epub.
Using the file manager I can see under the Kindle Documents folder, a directory of the authors name containing the large apnx file - the same structure as files which do appear in the Kindle library.
The book does not show in the library with all items enabled and a Kindle library search does not find it. If I delete the book on the Kindle using Calibre the directories disappear as expected.
I have completely removed all instances of the book and reloaded it and gone through the process again - same result.
I have also tried directly copying the files from my library onto the Kindle using file manager - same result.
I have restarted both Calibre and the Kindle, re-booted the PC - same result.
This is not related to the particular ebook as the same now happens with any attempted side load with Calibre or file manager.
I recently took the kindle out of aeroplane mode and it was updated without my consent to S.16.2.1.2. Not sure if this has caused the issue, but it has only happened after that update.
I have just successfully used the 'sendtokindle' website and the Amazon email option but that is not how I want to operate as I do not want to turn off aeroplane mode or rely on Amazon.
Internet and forum searches have not identified this problem that I can find.
I have run out of ideas and would really appreciate any assistance.
Please! Thank you.