This video demonstrates problem: https://saucelabs.com/beta/tests/d6529f0489274febbf9979e1f4b3e800/commands#255
Not sure if issue with beta cluster or software, but I suspect CentralAuth.
In Chrome browser
Register a new account on https://en.m.wikipedia.beta.wmflabs.org/
Visit a new page and click edit
Add some text
Inspecting the console I see an error relating to not being able to create while anonymous
A refresh of the page shows me as logged out
Refreshing again I am logged in
I think something in CentralAuth is not behaving here as it should.
This issue is causing MobileFrontend's browser tests to occasionally fail: