This wasn't an issue with my old J3-2016.
On the S21a:
If I turn the torch on first followed by the camera, the torch gets switched off.
If I turn the camera on first and try the torch, I get a notice saying...
Can't turn on torch because the light is being used by another app.
I know a workaround is to use video mode, which allows the torch to be on, and then use single frame capture, but this is not an ideal solution.