This option is like the existing --complete-graph-if-new-ver option, but it will trigger --complete-graph mode whenever the USE of an installed package will change (including USE changes suggested by --autounmask).
This is fixed in git: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=c42591b5d0484459837da28482f70fad9ff13824
Fix it to work with --autounmask: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=fdda543b51dde02483a40b9f867a35fc3521a135
This is fixed in 2.1.11.6 and 2.2.0_alpha117.