For the MI24P, as that was my test subject and has only two hit points with dependency:

1. Set the damage on a vehicle: _vehicle setDamage 0.8
2. Repair every hit point by looping through each value of getAllHitPointsDamage _vehicle and executing the following code:

{
    _vehicle setHitIndex [_forEachIndex, 0];
} foreach (getAllHitPointsDamage _vehicle) select 0;

3. Observe that getAllHitPointsDamage _vehicle now returns 0 for all hit points and that all 6 hit point boxes in the vehicle are white.
4. Use [_vehicle, "hithull", 0, true] call BIS_fnc_setHitPointDamage; and observe that none of the boxes have changed colour in the vehicle, and all values from getAllHitPointsDamage _vehicle are still 0.
5. use [_vehicle, "hitengine", 0, true] call BIS_fnc_setHitPointDamage; and observe that the hull is now red in the vehicle and the value for hithull has changed to 0.8 when checking getAllHitPointsDamage _vehicle or _vehicle getHitPointDamage "hithull".

Part of this is related to T120242 since "hithull" can be set, but without using setDamage to set the "Total" damage lower than "hithull", "hithull" will always reset to the value of damage _vehicle. That alone is very annoying, because setDamage sets damage over all hitpoints when you only want to change the "hithull" hitpoint.

If I am correct in assuming that the dependency means that the hitPoint will get set to the maximum of the hitpoint and its dependency, this would be the fix: (excluding defines and includes that would be before the params)

params ["_entity", "_hitPoint", "_damage", ["_useDependency", true, [true]]];

_entity setHitPointDamage [_hitPoint, _damage];

if (_useDependency) then 
{
	private _cfgHitPoints = configFile >> "CfgVehicles" >> typeOf _entity >> "HitPoints";

	private _expr = getText (_cfgHitPoints >> _hitPoint >> "depends");
	
	{
		private _res = _x call
		{
			if (!isNull (_cfgHitPoints >> _this)) exitWith {_entity getHitPointDamage _this};
			if (_this == "Total") exitWith {damage _entity};
			nil
		};
		
		if (!isNil "_res") then
		{
			_expr find _x call
			{
				_expr = [_expr select [0, _this], _res, _expr select [_this + count _x]] joinString "";
			};
		};
	}
	forEach (_expr splitString ESCAPE_CHARS);

	configName _x call
	{
		_entity setHitPointDamage [_this, (_entity getHitPointDamage _this) max call compile _expr];
	};
};