I was able to identify the issue.
mpc_pos_mode - acceleration based doesn't call Collision Prevention in any of the newest PX4 builds..
You need to use mpc_pos_mode - Simple Position Control (0) or mpc_pos_mode - smooth position control (3).
I was able to test this on a similar setup, and confirmed that collision prevention only worked with the mpc_pos_mode indicated above, and notably did not with mpc_pos_mode acceleration.
Can you see if you are using the MO169 or the MO178?
What TOF sensor are you using for obstacle distance?
Just confirming, you are using the ARK Flow MR facing downwards just for optical flow and position hold, but not for collision prevention?
Looking through the PX4 codebase, the ARK Flow MR publishes to distance_sensor by default, not obstacle_distance. Have you made any changes to the codebase that would affect obstacle_distance or mounted any other sensors?
Also, how have you mounted the ark_flow_mr. Is it pointing downwards on the vehicle, or forward pointing or some other configuration?
Finally, have you tried setting the distance for collision prevention higher, and seeing how the drone reacts in that case?