Hi Heinz-Jürgen, Thanks for debugging and submitting a fix for this problem!
In general, when proposing changes to the groff code base, it's best to open a bug report or feature request at: http://savannah.gnu.org/bugs/?group=groff&func=additem (You can submit an item here without creating an account, though if you do register, the tracker will automatically email you updates.) This is better because groff developer time is in short supply, so if no one applies your fix right away, a mailing list post will eventually fade into obscurity, but an item in the bug tracker will remain visible until someone can address it. Regarding the specific change you've proposed, there may be some resistance to using a grep option that's not part of the POSIX standard for the command. I'm not sure how widely implemented -a is, or what equivalent solution might be more portable.
