This is piling another workaround on top of an existing hack. I know I suggested the original version of this hack, so I'm guilty too ;), but let's solve this properly. Doing full parsing of all man pages every time one of them is updates is just too expensive. Also, making the mandb call asynchronous makes it necessary to retry (e.g. if the machine is rebooted too quickly), and makes the whole thing much more complicated. It is also always possible to schedule this job at inopportune time, taking 100% CPU for a few minutes.

Let's just do the minimum amount of work necessary, synchronously:

1. Add mode where mandb adds to the database. mandb has -f switch, but that disables database creation. A new switch like that is needed, that does not disable -c. Let's call it --update-path.
2. It must be also possible to remove existing paths. It would be simplest if --update-path also accepted a filename of a removed man page, and would simply remove this path from the database.
3. Add scriptlets:

%transfiletriggerun -- /usr/share/man/
mkdir -p "%{_localstatedir}/lib/rpm-state/man"
cat >"%{_localstatedir}/lib/rpm-state/man/uninstalled"

%transfiletriggerin -- /usr/share/man/
mkdir -p "%{_localstatedir}/lib/rpm-state/man"
cat >"%{_localstatedir}/lib/rpm-state/man/installed"
mandb --update-path $(cat "%{_localstatedir}/lib/rpm-state/man/uninstalled" "%{_localstatedir}/lib/rpm-state/man/installed"|sort -u)
rm -r "%{_localstatedir}/lib/rpm-state/man"

This is a POC and obviously untested, but I think this approach is much better. There is a list of uninstalled paths, and a list of installed paths. Some files might be on both. But mandb knows exactly which paths were touched and need to be adjusted.

Thanks, your suggestion is much better. I'll see what I can do about mandb.