That left/right cursor key bug is the worst. I'd assumed there was just something wrong with my keyboard until my colleagues also reported seeing it.
I often use _phrase_ to italisise. This seems to trigger the left/right arrow key bug semi-regularly. This fix is to find the Teams icon in the Taskbar, right click and Quit.
Then restart Teams.
> find the Teams icon in the Taskbar, right click and Quit.
...and never open Teams again!