On my iPhone 8 Plus with the newest IOS and Lightroom, the Lightroom Camera flash cannot be enabled (icon is disabled) if I am shooting DNG in Auto or Professional mode. For JPG the flash can be enabled (icon is enabled).
In the Camera App everything works fine.
IOS 11.3 iPhone 8 Plus Lightroom V.3.2.0 C96EDE
I tried reinstalling lightroom but this did not solve the problem