Hi Kleos,
Thank you for visiting Apple Support Communities.
So the flash setting of your iPhone sometimes doesn't change until you quit and relaunch the camera app.
Do you use the flash a lot? That could disable it for a bit.
Get help with the camera on your iPhone, iPad, or iPod touch - Apple Support
If the flash isn’t working, or if it’s going off every time you take a photo, check your flash settings. Tap the lightning bolt in the upper left of the screen to choose a different setting.
If you take extended video with the flash on, or you’re using the device in a very warm environment, you might see (disabled flash icon). You won’t be able to use the flash until your device cools down.
If the flash is on when you’re not using the camera, check the Control Center to see if the flashlight is on.
Take care,
Nubz