AC3.5.5 Pix+FBL Acro & Stable mode the swash plate will slowly collapse

A long time ago, once using the AC3.4.3 firmware to debug the Pix+FBL combination, when testing on my T-rex 500Pro DFC, there was a problem that the swash plate would slowly collapse.

A few days ago, after updating to the AC3.5.5 firmware, the combination of Pix+FBL was tried again, and the debugging process was relatively smooth. However, there was still a problem that the swash plate would collapse. It was not possible to take off in the stable mode. Only the Acro mode was used. Corrected circumstances can only take off safely.

Where does this problem occur?