[shamanNS]

For Calibre generated Kindle book files Calibre puts its own GUID identifier as a value in place where for Kindle Store "retail" books would have an actual B___ ASIN id.
Because both ASIN & cdeType with value "EBOK" need to be set in order for Kindle to treat "sideloaded" book as a "Book" and not as a "Doc" (Doc = cdeType "PDOC") Calibre started doing that years ago.


Airplane mode isn't your problem. Missing files in /system/thumbnails is.

Or maybe you also have book files that are azw3 format + cdeType "PDOC" (you can check that also with MobiMetaEditor) + older Kindle firmware combination that IIRC would refuse to use even existing thumbnail image file.