Hey there, @OttoKerner asked me to take a look.
First - There use to be a bug where sometimes changing the name of a device did not reflect properly in Plex Web. Is it possible, the device is listed as something else on your account?
Second (more likely situation) - Since your sister is a managed user, that means you or she must have used your admin login to authenticate her iPhone originally. This should have added the device to your list, then added it to her user when she changed to her managed user. I think there is a bug/loop hole that if you, as the admin, remove the device from your account, it does not remove the access from your managed users. Is it possible you deleted her iPhone from your list previously and just forgot about it. That could explain why she has access but you don’t see it listed.
The workaround would be to switch back to your user account on the iPhone, then log out. Since managed users cannot log back in, they will not have access without your admin login info.
We will look into this bug/loop hole.
