I also shoot to preserve the full tonal range, which generally means preserving highlights. If nothing is clipped, then yes, lift the shadows, which I find quite easy with filmic. If shadows are clipped, I make bracketed shots, which I generally then process with the base curve.
When I have an unclipped shot in one file, filmic is great.
There is another quite long filmic thread where @anon41087856 explained a lot of the math. If you are maths inclined, I’d suggest you look at that thread to see if you can spot the error you’re speaking about. If the error isn’t in that math, then perhaps there is an error in the code; but you’d have to read the code for that. The thread is: Introducing the filmic module in darktable