That's an OPEX behavior, I imagine.
You could try that (no guarantee):
in initPlayerlocal.sqf , recommended, (or where player is already defined):
player addEventHandler ["GetInMan", {
params ["_unit", "_role", "_vehicle", "_turret"];
_unit spawn {sleep 1; _this setCaptive FALSE};
}];
or in marker area (you can place a marker area and set it invisible (editor or _marker setMarkerAlpha 0) si you don't want to see it on map:
waitUntil {sleep 1; {_x inArea _marker} count allMines >= 20};