Never mind, it's the Focus Enhancer setting. I can disable this and the flash turns back on.
I've got a problem with my S21 Ultra. When I go into a low-light setting the flash option on the camera becomes disabled, and is not interactable. This is very frustrating: I'm trying to take a picture of my energy meters in a cupboard and when I move my phone into the cupboard the flash turns off, and the picture is useless.
If I turn on Pro mode then the flash stays on.
I've fiddled with the settings, including the scene optimiser and scene suggestion settings, but none seem to stop this issue.