Within the Modern UX, a Channel has been set up to link to a portlet in the Classic UI.
When using the Impersonation feature, the Classic UI Channel displays with the rights of the Impersonator, not the Impersonatee.
For example, if UserA has rights to see portlet ABC, but UserB doesn't, if UserA is impersonating UserB, they will still be able to see the information in the Classic UI.
Clarity 16.3.1
This is working as expected. The Impersonation feature does not carry to the Classic UI.
When a Channel is created, it provides a portal into the Classic UI, but no impersonation information is passed. It cannot be passed, as the Classic UI doesn't allow for impersonation.