Indeed, at small downward angle after max speed reached the UAV starts rising. Interesting bug but it is burred deep in physics simulation so not fixable at this time.

If set last value in envelope[]={}; config entry less or equal to 1 it will not climb. But there is still "bump" at 125% maxSpeed. It will descend but rate will be less than at 120% maxSpeed. Also speed display will not turn red.

Tested on B_UAV_05_F and UAV_02_base_F

uav setVelocityModelSpace [0,700,0];
onEachFrame {systemChat str (velocity uav)}

class CfgVehicles {
    class UAV;
    class UAV_05_Base_F: UAV {
        envelope[]={0,0.28,1.04,2.1600001,3.2,4,4.3200002,5.1199999,5.4899998,5.8800001,1};
    };
};

I’ve passed your ticket to our config person let’s see what they say