fiwiki VPT has a report of user in the reviewer group not being able to stabilize pages despite having the stablesettings right.
@Urbanecm tested this on a testing account and managed to reproduce. MediaWiki thinks that the user has the stablesettings right:
>>> User::newFromName('Martin Urbanec (test)')->isAllowed('stablesettings') => true
this sounds similar to T273317: some users with access are unable to configure pending changes but we suspect this is a different bug