If this thread's title doesn't specify it's spoilery, don't bring any up.
To all admin:
DaNASCAT and I have at last found the bug in the welcoming message system which has prevented it from working as advertised at help:Welcome tool and user:Wikia.
For the time being, however, we are deliberately leaving the welcoming feature operating in an erroneous way, so that the development team can clearly see the bug. It will be put back in proper order in a week or so.
In the meantime, this will mean that new users, be they IP or registered, will be greeted by the bog-standard default staff greeting. Though undesirable, this is necessary in order to squash this bug for all users of the Wikia platform.
Please do not change any pages that have the word welcome in them. This means pages like MediaWiki:Welcome-message-anon, MediaWiki:Welcome-message-user or the like. Also, please do not create any red-linked pages in the MediaWiki namespace containing the word welcome.
Update
Wikia have now publicly announced this issue is a problem in the latest Tech Update. This means, effectively, that it is serious, and that they're actually working on it. DaNASCAT has given me the all clear to do a temporary workaround so that we can get back to having more or less "normal", custom welcome messages. This workaround will not follow the instructions given at user:Wikia. But it will restore welcome messages.
A fix should still be coming in the next week or two that will make things work as they always should have done. At that point, the workaround solution will be deleted in favour of the "Proper Way of Doing Things". :)