Up and down arrow keys not working in Messages on MacBook Air (M1)

After the recent update to Big Sur 11.5.1 on my MacBook Air (with Apple M1 chip), the up and down arrows are no longer working in Messages, neither in the message list(left), nor in the message body(right). As soon as I press the arrow keys, the mouse pointer disappears; it reappears after I move the mouse with either a mouse or the trackpad. Unlike others described in other threads, I don't have a second screen set up, it's just the built-in monitor.

Is there a fix for this?