I noticed that if I begin to grade a LogC QT or Log DPX seq. Lumetri will not allow for any overbrights. Using an aggressive S curve or even a LUT everything is clamped 0-1.
If you have a Log File, import it, use a curve to reveal the over brights seen in the info panel. Turn off the stock "Curve" and now apply Lumetri, use its Curves and do the same S curve. All the over brights are clamped in the info panel.
Another test, set the exposure to 5 in Lumetri, apply another instance of Lumetri again and set the exposure to -5.
Everything is clamped and muddy, no round trip, it seems like this is not even a 32 bit float plugin?