I WAS WRONG!
I NEVER would have thought this was the cause, but I was wrong and I thought you deserved an update. I cannot explain why so many people were confirming that their phones would refuse to stay off... but I guess that was all just one giant red herring.
I was having trouble getting my USB-C cable to charge my phone, I would have to keep it at an odd angle in order for it to charge. I got tired of this, so I decided to clean out the USB port. I shut off my phone, grabbed a sewing needle, and then removed all the pocket lint that had collected in the USB-C port on my phone. After that, I plugged it in to charge and... it didn't turn on. Before today, it would power on the instant I plugged it in. Now, it stays off! I cannot tell you how much I HATE being wrong, but I also didn't want to leave you with an incorrect answer.
In fact... now that I think of it, all of the Android phones that people are "confirming" will automatically turn back on, they all have USB-C. At the risk of being wrong again, I wonder if it's a combination of some people just having dirty USB ports and some phones genuinely having that feature.
I apologize for misleading you. All of the data I painstakingly collected is now suspect. I am 100% certain that at least some modern phones have this "feature" but it is clear to me that I was completely wrong in my assertion that this was a new feature being pushed by Google... IT seemed obvious based on everything I was reading...
At least this explains why I couldn't find any official documentation on it...
I doubt your charging port is dirty since it is a new phone, but it is worth trying a different cable, a different charger, and if all else fails, cleaning the port anyway. Please downvote my previous answer so I do not accidentally mislead anyone. I will post a comment under that answer saying I was wrong, that way I don't confuse future readers