User blog comment:Rappy 4187/Technical Update: September 7, 2015/@comment-3388044-20150909041557/@comment-3388044-20150909061734

UPDATE: The issue seems to be not whether the wiki is a MessageWall wiki versus a UserTalkpage wiki. Rather, the difference seems related to whether a wiki is a busy one or not.

Wookieepedia is manifesting the same problem and that wiki has UserTalkpages, but it is a very busy wiki.