gamin provides libfam0. kcoreaddons should load just fine with libfam0 from gamin.
I did the research in #510368 and #966273, reviewing the actual code and confidentally concluded that FAM can be removed from Bullseye. The safest choice is to have a single library (gamin) used in the distro, rather than having both FAM and gamin. Please pick up the trivial change in https://salsa.debian.org/qt-kde-team/kde/kcoreaddons/-/merge_requests/3 so that kcoreaddons is not blocking progress. Thank you. Glenn