Thanks…
Cross product between unit vectors has magnitude 0 to 1. That should tell you what is going on…
So using only cross product there is no way for the curve tilt to tell how many revolutions have gone by so when the curve completes a “loop” it resets to 0… hence the 1 turn twist when that happens - and that twist will always be the same size as your curve resolution where the “reset” happens.
You’d need to keep track of how many turns your custom normal has made and add that (in units of tau) to the curve tilt also.
Can’t really help you with that as that will be heavily dependent on your setup.
Hope that helps.