Core answer
Start here. This is the direct spoken answer to practice first.
Overview
Forcing a render in a test can make stale production state look correct even when Angular was never notified of the change.
In zoneless Angular, rendering is scheduled by explicit notifications such as updating a signal, setting an input, handling a template event, or marking a view for check. A test that mutates a plain field and immediately calls detectChanges bypasses that scheduling requirement. I reproduce the production mode, perform the real state transition, await whenStable, and fix the missing notification.