I've never seen the behavior you are describing. Whoever posts a message it comes from that person if it's a standard Teams channel message. Only time I've seen owner / creator of Team show up is adding / removing people from a Team outside of Teams itself through azure ad etc.
Also when you post a message you don't get notifications for your own messages. It sounds like you have a Power Automate on the Team created by someone that's monitoring messages and sending out notifications. I would get with that Owner / Creator of the Team and check his Power Automate flows for a Teams trigger causing this which my gut feeling seems to be the case here.
