kholidays formerly came with kde-apps, now it seems it got moved to kde-frameworks (with adjusted version). It came with kdeapps-17.12.1 which just got treecleaned. The last available version seems to be now 17.08.3, which portage wants to downgrade to. I think at least plasma-5.12.0 should depend on the most recent version if stabilization plans allow it, alternatively restore kde-apps/kholidays-17.12.1.
I don't quite understand your problem. Plasma 5.12.0 accepts both {kde-apps,kde-frameworks}/kholidays and prefers the new one.
It does sound more like a keywording problem on your side.
FTR: $ grep kholidays kde-plasma/plasma-workspace/plasma-workspace-5.12.0.ebuild calendar? ( || ( $(add_frameworks_dep kholidays) $(add_kdeapps_dep kholidays) ) )
Thx, you are right. I 1) assumed it would be a package move for unstable plasma and 2) checked the 5.11.5 ebuild when I wanted to look at the 5.12... I was in a hurry this morning. Sorry for the noise!