When you are using TTL, the camera is responsible for measuring flash exposure and ending the flash once sufficient exposure is reached. It sounds like you are controlling flash exposure compensation from the camera rather than the flash (you don't say so explicitly, though). In that case, it would appear that the flash ends its light independently of camera control. There may be two reasons for that:
a) it has run out of energy: the camera wants more exposure but the flash has done all it can
b) it isn't interested in what the camera says because it is in automatic exposure mode, using its own scene metering. Which would mean that we are not actually talking about "TTL" at all here.
Note that even in automatic metering, the camera may be communicating ISO and aperture to the flash (and possibly effective zoom length) so that metering will work comparatively well. However, the kind of difference in behavior when a strong reflector is just inside of the frame or just outside will not be achievable.
Also, most of the time when this kind of communication level works out, actual TTL would usually work as well as long as the flash itself is in TTL mode.
It should usually have some kind of display and/or mode switch telling you that.
What kind of flash is it?