On my Galaxy S21 Ultra it was an option to show the contents of a message notification or not such as a preview. Say you get a mesaage...a bubble will pop up saying "new message" or show a preview of the message. Now it says the person's name and the contents of the message and I don't know how to change it. I turned off notification previews but it doesn't affect the message preview. This issue has yet to be addressed correctly on YouTube so far...I have only found videos showing how to turn off previews which doesn't turn off message previews.
Try going to settings then notifications and then lock screen notifications. In there you can set which apps can show notifications when phone is locked or you can hide it all together.
