*ERROR* Async flips aren't allowed

I found that adding ,reflect_x to /boot/cmdline.txt renders errors in dmesg (/var/log/syslog). However, I need to reflect the screen. This is running DietPi Lite.

On 1 RPi4, removing video=HDMI-A-1:1920x1080@60,reflect_x (which performs the horizontal flip required for our setup) stops the error messages. On a 2nd RPi4, remove video=HDMI-A-1:1920x1080@60,reflect_x has no effect on this error message.