The "sunny" quick setting is hardcoded to 2 values: 100 and 0. While 100% for sunny mode is OK, setting the brightness all the way down to 0% for non-sunny mode makes the screen completely black and the OS unusable (unless you remember where the button was so you can turn it back to sunny and then set a reasonable brightness through the full settings – if you do not, you might even think the OS has crashed altogether) at least on the LG G Watch R (lenok), and probably on some other watches as well.

The full settings actually do not allow setting the brightness below 10% (presumably for this very reason), so at the very least the quick setting should also use 10% instead of 0%. A smarter approach might be to remember the brightness that was set before sunny mode was turned on and switch back to that. But just changing the hardcoded value from 0 to 10 would be a very quick fix.