@jdc youre right, i was talking about auto-matched Tone Curve in Exposure , but after checking again, i cannot reproduce this behaviour myself.
My bad: I was playing on Bring Life in a flat background - #10 by Jean-Marc_Digne which is a dng-file and the auto-matched tone-curve doesnt work with any version. Its just strange that an applied manual curve is reset by this button, but thats not your concern.
pls take my apologies for confusing you
Martin