This is not a bug, it was implemented that way on purpose. Only --sync pays attention to replaces or provides fields, since they rely on the sync database. -U and -A operations do not rely on a sync database, so they can be used offline.

Well honestly it is an odd implementation. I would have never assumed this behaviour unless I have read it here. I guess that some note when using pacman with -U or -A flags should be added to the pacman log when the package contains a replace line.
Still I do not see the point for such behaviour, as at least in normal installations one always have access to the local pacman database and therefore you can check if the replaces affect anything in your system.