Flight mode change failed when use CX-OF(optical flow)

Frame: x4 multicopter
Flight Controller: pixhawk
Firmware: ArduCopter V3.6.9 (632be63f), PX4(Nuttx)

Recently I’m using loiter mode indoors with a optical flow(CX-OF). I have followed the instructions in the wiki(http://ardupilot.org/copter/docs/common-cheerson-cxof.html, http://ardupilot.org/copter/docs/common-px4flow-overview.html) such as configuring parameters and calibrating the sensor but it doesn’t work. Pixhawk took off in AltHold mode and then I tried to switch to loiter mode while it refused. I got the message(Flight mode change failed) and error(Subsys 10, Ecode 5) from the following log.
loiter-fail-cxof-20190724-02.zip (501.9 KB)

You may consider that the cx-of is broken or I don’t mount it in the right direction, so do I. But to my surprise, when I load the newest firmware( ArduCopter V3.7.0-dev (5b5f5823)), it works perfectly. Just changing the firmware and nothing about the hardware, it can switch to loiter and can hold the position.

I don’t know why it has problems with 3.6.9 while works with 3.7.0-dev, can anybody help me?

Can anybody help me?

This feature has been enabled since 3.6.9 rc2

  1. CX-OF flow sensor enabled on all boards

More details at this link

Thank you for replying.
The firmware of my pixhawk is V3.6.9 (632be63f) which is after 3.6.9 rc2. And my pixhawk can read data from cx-of. The problem is it can not switch to loiter mode.