Message saving does not work but the extended editor does not reflect it.
Fragment of a discussion from Translating talk:MediaWiki
The dialogs are cached, which is the reason you see the text when opening it again.
How about improving the "invalid token" to something directing the user to a more helpful action? Such as: "invalid token. Your edit is not saved, but may be buffered locally. You need to open a fresh edit window; then copy your edit to it, and save it if you wish."