See T415899: Mobile editing anon warnings are not shown on auth pages, that bug actually looks like it's caused by CentralAuth in SUL3 mode.
Although the discussion in that task leans towards intentionally removing this particular warning, the same feature is used by other code, and should be fixed. Special:Watchlist is a simple example:
- Visit https://meta.wikimedia.org/wiki/Special:Watchlist while logged out
- It redirects to https://meta.wikimedia.org/w/index.php?title=Special:UserLogin&returnto=Special%3AWatchlist&returntoquery=&warning=watchlistanontext
- …which redirects to https://auth.wikimedia.org/metawiki/wiki/Special:UserLogin?useformat=desktop&usesul3=1&returnto=Special%3AWatchlist&returntoquery= – note the 'warning' parameter is gone