The exponential.aif file is a true exponential function in that it begins with a small non-zero value. The exponrev.aif was intentionally designed or forced to end on a zero.
If you'd like a FunctionGenerator amplitude envelope that always lets a little signal through, even when not triggered, you could use the WaveEditor to generate (or modify an existing) wavetable such that the final sample value is small but not zero. Alternatively, you can add a small DC offset to the signal flow using a ScaleAndOffset with the Scale set to 1 and the Offset set to the small DC offset value.
Since the resting state of the FunctionGenerator is on the last sample of its Wavetable, when you put it into the Product you are multiplying your input by its final sample during the "resting" phase (before you trigger it).