Steps to reproduce:

• Login as admin
• Create a managed cohort. These are cohorts which have an entry != null in the 'component' column of the 'mdl_cohort' table. These cohorts are managed by a plugin and cannot be managed manually by an admin. If you do not know how to create such a managed cohort in a realistic way, you can use these steps:
• Install the https://moodle.org/plugins/local_profilecohort plugin
• Go to Site administration -> Cohorts
• Add a new cohort with the name 'Managed cohort'
• Go to Site administration -> Users -> User profile fields
• Add a new 'text input' profile field with an arbitrary name
• Go to Site administration -> Profile field based cohort membership
• Click "Add new rule"
• Select the user profile field from the dropdown and configure an arbitrary rule
• Click "Save changes"
• Go to Site administration -> Cohorts
• Verify that the 'Source' column of your cohort row says 'Profile field based cohort membership' and not 'Created manually'
• Add another cohort with the name "Manual cohort'
• Have a look at the actions column (the last column) in the cohort list

Expected result:

• The 'Manual cohort' has an action menu with the Menu items Hide, Edit, Delete and Assign.
• The 'Managed cohort' does not have any action menu as it is fully managed

Actual result:

• The 'Manual cohort' has an action menu with the Menu items Hide, Edit, Delete and Assign.
• The 'Managed cohort' has an action menu with the same menu items as well.
• If you click any of these action menu items, you are redirected to the same page again but nothing else seems to have happened.

Regression analysis:
In Moodle 3, managed cohorts did not show any menu items in the cohorts list and appeared as fully managed.
Against this background, I would say that this is a regression in Moodle 4.