Right now, when a review is requested the targeted reviewers are "assigned" to that PR.
This is due to legacy reasons, as in the past there was no "reviewers" metadata for the PR.
Now, instead of assigning a PR to the reviewers, the reviewers should be assigned via the dedicated field.
In summary:
- The "Assignees" field should contain the list of PR drivers. Persons who will make changes to get the PR approved.
- The "Reviewers" filed should contain the list of PR reviewers. Persons who should check the changes and approve or reject them.
Here are the 2 fields.

Note that this is only valid for PRs.
For issues, we will continue to have the existing behavior via "Assignees"