on the sideloads disappearing from books folder workaround

[stumped]

i was reading all the CC FAQ yesterday because of another issue, and saw the " don't send books to the books folder" write up there.

but i wanted an azw3 on my fire tablet yesterday, so I asked CC to fetch it, it was delivered to "books" because I never bothered to change that to "kindle". and it appeared OK in the books tab.
And the book I downloaded a couple of months back is also still ok

So the claims that Fire OS moves/deletes stuff sent to "books" does not seem to happen on my tablet. - I checked in settings and it is running fire OS 5.6.10 updated Jan 19
So is the suggested change to use "kindle" folder only relevant to devices running fire OS 6+ ?
I am mentioning it in case the FAQ entry needs tweaking or that more testing needs to be done before concluding that amazon definitely moves stuff
the 2 books I have sideloaded are large AZW3 game guides
I could fetch more/ different books as a test if that would help any
and/or: I have a 2017 fire HD 10 that I am not currently using ( which may have fire OS 6+ - I would have to check ) : I could fetch those same guides using that, to its books folder and see if they vanish or not