Found when verifying https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/306560 for T143837: Append mw.notify div to the body instead of content so it is always on top.
CX page displays the session expire warning:
Note: the skin was Monobook - not sure whether it's relevant.
With the new version of Content translation this seems to be fixed:
More details in T200646#4549952