Clarification on Reviewer Access in worksheet

Summary:

We have a requirement to assign both a secondary reviewer and other reviewer for a worksheet, with different access levels:

The secondary reviewer should have update access.
The other reviewer is expected to have read-only access (no updates allowed).

However, we are observing the following behavior:

When acting as a proxy for the other reviewer, the worksheet is read-only.
But when logging in directly as the other reviewer, the worksheet is updatable.

We would like to confirm:

Is this expected system behavior?
Or, by default, does the other reviewer also have update access when logged in directly?

Content (please ensure you mask any confidential information):