Suddenly CTUN.Thl increased while RCIN.C3 is at minimum

I was flying a drone indoors without a GPS. rangefinder is TFmini-s connected to the pixhwark running copter4.4.0. I’m having an issue where the quadcopter crashes hits the ceiling in AltHold mode (suddenly altitude jump on) when I checked the logs seems there’s a periods of where the rangefinder disappears for a few seconds then came back but this behavior occurs after changing flight mode from Land to Althold mode.

Seems CTUN.Alt (the current altitude) drops and CTUN.Thl (the desired throttle level) increases, the flight controller is trying to compensate for an unexpected descent. indicates that the drone is losing altitude and the flight controller is responding by increasing the throttle to regain or maintain the desired altitude.

@ rmackay9 What caused the copter altitude suddenly Descended? CTUN.Alt to drop suddenly.

The following shows logs of altitude.
Flight logs attached here

Screenshoots: