On my FP3, also Android 13 and the same version number as you have (6.A.023.1), I can use the “slow-motion” mode without any problem. The version of the camera application I have is 3.0.002.
Have you tried clearing the application cache? You can do it from:
Settings → Apps → All apps → Camera → Storage & Cache → Clear Storage & Clear Cache
Also, from the camera app itself, by going into settings (gear icon at the top left of the screen), you can try clicking on “Restore defaults”.
Hopefully one of these methods will get slow-motion mode working again… if not, another thing I can think of: are you using the default camera module or have you upgraded to the “+” module? Previously the Slow-Motion mode worked fine? 
Good luck! 