If /etc/portage/package.mask is a directory, emerge will concatenate all their content, resulting in completely unrelated comment lines preceding a given atom. The source is only given as /etc/portage/package.mask, the actual file inside that directory is not named. I'll attach a patch to improve this situation by splitting comments at file boundaries, and by printing the actual file name instead of the directory name.
Created attachment 306183 [details, diff] Proposed fix v1
Thanks, this is in git: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=cf7bf7667a56a28a6c6f360bc77a01d05d64bdd8
This is fixed in 2.1.10.50 and 2.2.0_alpha94.