I think this is blaming the user. Math does not “break”, that module (like many others in Darktable) simply uses an interpolation that introduces spurious oscillations into the actual curve, unexpected by the user.
The cleanest approach would be replacing these interpolations with monotonic ones, which are in accordance with user expectations.
A quick fix would be a visual warning when oscillations happen (eg like AgX highlighting parts of the curve in yellow when convexity expectations are violated, eg shoulder being concave).