The vary-revision parser flag doesn't appear to be working consistently on the WMF production cluster (I tried on both enwiki and mediawikiwiki). As far as I can tell, this problem doesn't happen on the beta cluster or my own testing instance. To reproduce, create a page with something like this:
(replacing "User:Jackmcbarn/Sandbox2" with the name of the page). Then, change "aaa" to "bbb" and save. Occasionally, instead of the correct "bbb bbb", "aaa bbb" will appear.