The functionality of DMI (Direct Message Injection), although it is designed to bypass other security tools and into the user's inbox, prevents the Google warning gray banner from being displayed.
Phishing and Ransomware simulations are tests that are part of reality, and it is possible that in many emails, Google will display that warning, which still serves to measure user behavior.
Something that can be tried is to use different content or scenario. Depending on how Google analyzes it at that moment, it may classify it as Spam or not.
According to what is indicated in Google's documentation, the banner will continue to appear if Google decides to mark the email as Spam.
We recommend viewing the following article and video:
Do you really know what a phishing simulation is? Are you sure?