https://pt.wikipedia.org/wiki/Special:ListUsers/confirmed does load the list of confirmed users for me when logged under my main account (Martin Urbanec). However, it does not work when I log out, or when I switch to Martin Urbanec (WMF) or Martin Urbanec (test).
Observed behavior (from some accounts)
I normally use timeless as my skin (for both my main and WMF accounts), but I switched to Vector. It is broken there as well.
I asked @Tks4Fish if they can reproduce this issue, and it doesn't work for them logged in or logged out. Even weirder, it shows push notification managers for them logged out, while that group isn't supposed to exist in ptwiki at all.
Screenshot by @Tks4Fish
I'm honestly unable to provide any theories, this is a quite weird bug. @Pchelolo maybe relevant to recent UserGroupManager refactoring?
See also T275310 for an user-created task, which made me to notice this bug.