The cubic spline interpolation will most likely become unstable and hit the ground before 0 for grey = 18 % with gamma = 1.0, so what you see is probably just numerical artifacts that shouldn’t happen.
BTW, I have developped a custom new spline from scratch here : https://eng.aurelienpierre.com/2018/11/30/filmic-darktable-and-the-quest-of-the-hdr-tone-mapping/#filmic_tone_curve . It should behave better (that is : properly) in the dark area with gamma = 1.0. @hanatos is testing it, we will see here it goes. (If this child gets his brain and my gorgeous looks, he shouldn’t need a fairy godmother).
Also, please, trust your eyeballs and stop looking at numbers as if they held some kind of truth. These numbers are there only for those who use lightmeters and can then translate their scene reading directly into parameters, but for the rest of the folks, the parameters will depend on what you did previously in your pixelpipe.
For example, there is another way to use filmic : set up your exposure until the midtones look bright enough, and don’t pay attention to the highlights that you will blow up. Then, adjust the white exposure in filmic to recover them. Since your white exposure will then be around - black exposure (that is, white between +4 and +6 EV, black between - 6 and -4 EV), the curve will be more centered and the spline should behave even for gamma = 1.0. But you might want to add some saturation in chroma preservaction then, because it tends to wash colours (which is supposed to respect the Abney effect, so it’s ok maybe ?).
Good thing I didn’t hardcode that gamma, isn’t it ?