You lead me to a good path, thanks.
If I look to the RCOUT graph (C2-C7), I can see that at 17:39:25, C7 begins to rise and C2, by compensating, begins to sink. At 17:39:31, C7 reach its max and will not move from there, C2 staying low.
As I stated before, I found the hexa with a missing propeller on MOT 01 (C7). The graph seems to confirm the inflight loss of propeller on C7 that began to turn on its max to compensate the loss of portability. C2 compensated to keep the hexa levelled.
I will go back to the flying place to try to find the propeller back.