This corresponds to reports in January that clicking the pencil produced the old message requiring login, that purge fixed the issue on the individual page and that on January 24 the issue seemed gone.
I had discussed the matter briefly with bblack on #wikimedia-operations but we were unable to raise actionable evidence. I note that on January 9 there was a series of deployments for MobileFrontend, so it's possible that something happened.
How to prevent this from happening again? Also, is it happening again now, as the decrease in edits after the latest deployment (involving 22e06675dbd1d2630b0e602c782be35145e8b94d) suggests? Can that be verified by purging mobile caches on it.wiki now?