Automatic sprinkler valve will not shut off
Opening a drain valve downstream will not raise downstream pressure, it will increase flow and lower total system pressure.

If there is any flow the valve should close, as pressure above the diaphragm equalizes with supply pressure upon closing of the solenoid pilot port.

Get the brand and model # of the valve and order a rebuild kit for it as your problem is very likely to be mechanical. Turn off the supply and remove the valve bonnet, inspect and clean internal components (flush all ports to remove debris, check the diaphragm and support rings for damage or cracks, inspect the sealing surfaces and seats). Replace the washers and diaphragm with new ones from the rebuild kit.