faq-proposed is not in the list of tagsthe list of tags that were asked to be synced that led to this functionality in the first place, so my assumption is that tags outside of this predefined list are not synced between Meta and the per-site metas.
Since you can also not create tag wikis on per-site metas, you'll note that most tag wikis on per-site metas are blank across the network. There are some exceptions to this, as can be seen on Gaming's meta, but as evidenced by the wiki history this is either currently a moderator-only functionality, or simply occurred before this restriction was put into place. Based on a related bug reporta related bug report, I'm going to guess the former, although until that's fixed I suppose it doesn't make much difference.
If the syncing is indeed restricted to that specific list of tags, it would probably be sensible to allow meta tag wikis to be created at-will for everything else...and you should now be able toshould now be able to edit not only those, but also the ones from the list.