During testing of FL-9561, I discovered some ebuilds which are also afflicted by XDG_CACHE_HOME being leaked into the build environment, causing sandbox violations.
While FL-6926 is not resolved, I believe at least the ebuilds in question should each have its issue addressed. In my system, the ebuilds I managed to identify were:
x11-libs/pango
dev-python/pygobject
net-dns/avahi
app-crypt/libsecret
dev-libs/libgudev
dev-libs/libxmlb
x11-libs/bamf
media-libs/gegl
media-libs/libmypaint