Torcat, Gabriel Thanks for reaching to Microsoft Q&A. I have summarized our discussion and posting as Answer for the benefit of community.
By following the suggested steps and upon checking further you have confirmed that the initial issue is with git hub actions not with the deployment.
Also, I have understood that you are looking for PR based deployment in static webapps. Using Pull-request environment you can achieve your requirement.
Hope this helps.
Feel free to reach back to me if you have any further questions on this.