Just because it compiles fine, doesn't mean there aren't bugs. If gentoo does not consider qt5 stable, maybe this is a symptom. OTOH, Kovid builds calibre on gentoo so *presumably* it should be fine.
It does work fine on ArchLinux -- or would if they didn't patch out modified libs in calibre.
Either way if it cannot be duplicated in the binary release it isn't a problem in calibre itself and might not get fixed.