Unable to approve MR with 2 users when both push to branch

Summary

Unable to approve merge request with 2 users in a project with 1 approval required and self-approval enabled.

Steps to reproduce

1. Create a project with 2 users requiring 1 approval with self-approval enabled.
2. Create MR as user A.
3. User B pushes to the same branch that user A opened the MR with.
4. Neither user is able to approve the merge request.

What is the current bug behavior?

Unable to approve the merge request without adding a new user to the project. Users are not allowed to self approve.

What is the expected correct behavior?

Users should be able to self approve.

Customer ticket: https://gitlab.zendesk.com/agent/tickets/115381 (internal use)