For a long time Microsoft displayed your newest channel posts at the bottom of the screen and to create a new post you would click the “Start a new post” button at the bottom of the screen.
More recently Microsoft has decided that it would make more sense to turn things around, moving the “start a new post” option and the newest content to the top of the screen. This has caused some confusion since it was the opposite of how it worked previously 
The good news is you can now choose for yourself which is best 
To switch between the two options at the top-right of your Teams app click … See new posts at top or … See new posts at bottom
Once selected the change applies to all Teams channels, not just the one you currently have selected.
