This sounds like a bug that I vaguely recall hearing about years ago. I wonder if there has been a regression. You might check to see if there are old reports of similar issues in Phabricator, probably from the Bugzilla era.
Pine
Hi,
We're trying to get more information about a bug that seem to have
caused a couple of users to get logged in as other users. If you have
two minutes to spare, there are a couple of sentences here that could
use translation so we can reach out in your language and ask if anyone
has encountered this.
https://meta.wikimedia.org/wiki/User:Johan_(WMF)/Session_pollution
//Johan Jönsson
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