Created attachment 532100 [details]
The result of [emerge -pvDNu world] changes randomly between runs.
My worldfile contains:
4.12.12 remains installed, but is no longer in the portage tree. Lacking any other option, portage wants to merge gentoo-sources-4.9.95::gentoo, the last currently stable version of gentoo-sources.
But it doesn't always do so. Running emerge multiple times in a row, without any config changes between, yields a roughly 50:50 chance that gentoo-sources-4.9.95 is among the list of merge candidates.
This is currently reproducible reliably.
Related forum thread: https://forums.gentoo.org/viewtopic-t-1081078.html
This occurs because the atoms from the world file are processed in random order.