After update to media-fonts/cantarell-0.0.22 german umlauts are rendered incorrect under harfbuzz-1.2.0. The Problem is already known and fixed upstream with 0.0.23. Ref. upstream https://github.com/GNOME/cantarell-fonts/blob/master/NEWS "0.0.23 - reworked anchors for correct accent rendering under harfbuzz 1.2.0"
Fixed, thanks for reporting! https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=a9e9888d9fb33985a8c3a141617cc11797496f77