I am familiar with the problem a white room can play with TTL flash, but I also notice when I get that familiar dark look to the subjects, plus that overly bright white wall behind them, I generally compensate by opening up a little, but in some cases the flash still does not seem to light the scene properly! ..I don't know if it is compensating for my compensation or what! ..has anyone else experienced this, and how do most of you handle the white room TTL problems.
Compensating by "opening up a little" don't work, because (as you say yourself) the magic of TTL "compensates" for that to keep exposure constant.
What you need to do is to use is FEC (Flash Exposure Compensation - Canon) or FOLC (Flash Output Level Compensation - Nikon), or whatever this feature is called on the system you prefer to use.
Read the fine manual .
– gisle [ See profile/plan for equipment list ]