Bitbucket 7.18 does not render PR overview client side extension correctly

Issue Summary

Bitbucket Server/DC 7.18 does not render PR overview client side extension className attribute.
This works in 7.17, 7.16,

Steps to Reproduce

https://community.developer.atlassian.com/t/bitbucket-server-dc-7-18-does-not-render-pr-overview-client-side-extension-correctly/54013

Expected Results

The extension's `className` attribute is rendered.

Actual Results

The `className` attribute is skipped.

Workaround

no known workaround