Conversations involving multiple participants, allowing collaboration and information sharing in Teams
I managed to fix this for myself by going to my colleague's message, hovering over as if i was going to react to it, reacting to it, going to the three dots on the react bar above their message and then pressing esc. Clicked back in the message box and suddenly my left-right functionality worked for me. I don't know if reacting to it is a necessary step in that but it worked first time for me so I didn't have a chance to test not reacting to my colleague's message. Hope this helps.