Teams does both - normally it’s Enter to submit and Shift+Enter for a new line, but when you open the formatting tools it switches. They at least do have a message indicating which key combo inputs a new line, but it still gets me on occasion.
The behavior also changes if you start editing a numbered or unordered list. Maybe that enters the "formatting tools" mode you mention?
I had managed to be on Slack exclusively for at least 10 years. Recent acquisition has me using Teams and it's hilarious to see for the first time what people have been complaining about. I thought surely people are exaggerating. No, no they are not.
It only took a couple weeks for me to figure out that I would have to compose longer messages somewhere else and then paste them into Teams.
MS is amazing in their ability to fuck shit up for no apparent reason. Like making a media player that doesn’t use space for play pause…
But if you click an arrow on the top of the text box, it expands to more than half of the height of the window, and now Enter does a line break and Shift-Enter sends. Which makes a lot of sense because now you're in "message composer" / "word processor" mode.