18.8.0: Merge Request reviewer dropdown crashes and does not send request

Summary

In GitLab Community Edition 18.8.0, assigning reviewers from the Merge Request sidebar fails. The reviewer dropdown spins indefinitely and no network request is sent.

Environment

• GitLab: Community Edition 18.8.0
• Deployment: Self-managed (single node)
• Load balancer: GCLB (Cloud CDN disabled)
• Browser: Chrome and Firefox (reproducible)

Steps to reproduce

1. Open any Merge Request
2. Click the Reviewers dropdown in the right sidebar
3. Select any reviewer (other than "Assign yourself")

Expected behavior

A request is sent and the reviewer is assigned.

Actual behavior

• UI shows a loading spinner forever
• No REST or GraphQL request is triggered
• Reviewer is not assigned

Browser console error

Uncaught (in promise) TypeError: this.currentSelectedReviewers.filter is not a function
    at updateReviewers (reviewer_dropdown.vue:303)
    at processReviewers (reviewer_dropdown.vue:289)