It's a mystery

because your test case shows the hand cursor I see in other apps.
The precompiled calibre binaries show the X11 one.
Maybe it is Qt-built-on-Ubuntu that does this wrong.

Didn't you once mention you freeze calibre on a debian VM these days?
(You
did test this on your Arch system using the frozen binaries for the sake of thoroughness? In that case, still a mystery?

)
Oh well, there are worse problems I suppose (for me at least

).