I was playing with my Canon 600D RT flash, my 7D2 camera, and the old Tamron 90mm f/2.8 Macro lens.
I was in my office at night with medium bright (that's a scientific term - didn't you know

) overhead lights. The exposure reading at ISO 200, is 1/25 second @ f/2.8.
Testing was done with the flash mounted on the camera hotshoe with the flash depressed. I used a Stofen diffuser straight on...
The camera/lens will expose O.K if I use Programmed mode on the camera and ETTL for the flash. That gives me 1/60 second @ f/8 in normally lit room. However, if I place the camera on manual at 1/60 second at f/8 with the flash on ETTL, the image is over exposed. Even when I dial in up to -3 stops of exposure compensation on the flash.
If I use aperture priority and ETTL, I cannot get a decent exposure, no matter what f/stop I select
I can get a decent exposure with the flash on Manual at 1/64 power and the camera @ f/22 and 1/250 second, I get a decent exposure but this is totally the flash exposure.