New version of net-print/hplip-3.18.12 can't be installed because net-print/hplip-plugin-3.18.10 depends on net-print/hplip-3.18.10. But this package have a new version 3.18.12 on HP developer site: https://developers.hp.com/hp-linux-imaging-and-printing/plugins. I think it's need to update.
I was going to put the same request.
For the time being, I'm running these commands after each --sync: # cp /usr/portage/net-print/hplip-plugin/hplip-plugin-3.18.10.ebuild /usr/portage/net-print/hplip-plugin/hplip-plugin-3.18.12.ebuild # ebuild /usr/portage/net-print/hplip-plugin/hplip-plugin-3.18.12.ebuild digest
This version already in tree