Have you tried to disable the FuseAllVelocities to your 18" propeller drone?
Have you also tried to change them?
EK3_RNG_USE_HGT, careful with SURFTRAK_MODE,1
EK3_RNG_USE_SPD
Do you think you faced the same situation as mine before?
I never use the rangefinder for height that comes with the optical flow, I always use an additional external rangefinder for height (parallel process). Also, the range is higher. onboard to me is like serial processing.