I can confirm the error but, it seems to be related to using spawn and createhashmapobject. And it only affects the constructor... I am guessing constructor runs either scheduled or unscheduled depending on the parent scope when it should probably always be unscheduled. So, for example, using [] call { ...code... } without the uiSleep doesn't produce the error -and- I am guessing methods always work because they are 'called'. as Biki states:

Methods (if they are called correctly, either by engine or via call - Syntax 3) will have the objects instance inside the "_self" variable.

So doing a '[] spawn { createhashmapobject [....] }' seems to be putting the constructor in a scheduled state (-- is my guess). When it should probably be unscheduled as it is called 'by the engine'?

Nice detective work. I also found only the CTOR affected, but - sadly - whether scheduled or unscheduled, the error persists. (The code in the attached map was my actual second attempt; the first was without spawn()).

initPlayerLocal.sqf is ran as scheduled iirc. If you had wrapped it in a call block, I believe it would run that block unscheduled.

Yeah, no dice. Tried it wrapped, tried w/ calling a setter instead of using set... All to no avail. Remains broken.

I thought I had it working with 'call' instead of 'spawn' (which it does work if called unscheduled) but, you are right. Call in initPlayerLocal.sqf is scheduled. I don't know why I thought it wasn't. So if you wrap it using isNil, it does fix it as a workaround. This does need to be fixed though.

Here is what I put in initPlayerLocal to get it to work:

uiSleep 5;

private _temporaryVehicle = [
    	["#create", {
    		params ["_vehicleType", "_vehiclePos", "_lifetimeSeconds"]; // handle constructor arguments
    		private _newVehicle = _vehicleType createVehicle _vehiclePos;

    		_self set ["MyVehicle", _newVehicle ]; // Store the vehicle inside the object for later

    		// because _self is passed as parameter, it will be referenced by the spawned script until it ends.
    		[_lifetimeSeconds, _self] spawn { params ["_lifetimeSeconds", "_self"]; sleep _lifetimeSeconds; };
    	}],
    	["#delete", {
    		deleteVehicle (_self get "MyVehicle"); // delete the vehicle when we go away
    	}],
    	["MyVehicle", objNull] // placeholder, this is not needed
];

isNil {  //Force unscheduled in a scheduled environment
    // create a temporary RoadCone, at player position, that will delete itself after 5 seconds.
    createHashMapObject [_temporaryVehicle, ["RoadCone_F", getPos player, 5]];
};

systemChat "DONE";

Just to clarify the issue, when createHashmapObject is invoked in a scheduled code block, the variable '_self' refers to the array passed to the command and NOT the created 'hashmapObject' in the '#create' constructor method but works fine if called from an unscheduled code block.
Methods called in scheduled code block are fine. '_self' refers to the object. For example:

private _temporaryVehicle = [
 	["MyConstructor", {
 		params ["_vehicleType", "_vehiclePos", "_lifetimeSeconds"]; // handle constructor arguments
 		private _newVehicle = _vehicleType createVehicle _vehiclePos;

 		_self set ["MyVehicle", _newVehicle ]; // Store the vehicle inside the object for later

 		// because _self is passed as parameter, it will be referenced by the spawned script until it ends.
 		[_lifetimeSeconds, _self] spawn { params ["_lifetimeSeconds", "_self"]; sleep _lifetimeSeconds; };
 	}],
 	["#delete", {
 		deleteVehicle (_self get "MyVehicle"); // delete the vehicle when we go away
 	}],
 	["MyVehicle", objNull] // placeholder, this is not needed
 ];

 // create a temporary RoadCone, at player position, that will delete itself after 5 seconds.
 MyObject = createHashMapObject [_temporaryVehicle];
 MyObject call ["MyConstructor",["RoadCone_F", getPos player, 5]];
 systemChat "DONE";

The constructor call environment depends on the caller.
its just a call. So if called from scheduled, it will also be scheduled. If called from unscheduled, it will

also be unscheduled.

initPlayerLocal.sqf is ran as scheduled iirc. If you had wrapped it in a call block, I believe it would run that block unscheduled.

no, call doesn't change the context/scheduled-ness

Wow. I indeed F'ed up the variable passing in scheduled execution.
Instead of the new map, it passes the argument that you passed to createHashMapObject.

So if you do this

createHashMapObject [_temporaryVehicle, ["RoadCone_F", getPos player, 5]];

in unscheduled (either when you force it via flag, or when you call the code from unscheduled script) _self will be the new map that's created.
If it executes in unscheduled, then _self is [_temporaryVehicle, ["RoadCone_F", getPos player, 5]];
That doesn't match your script error message... don't quite understand that part, the argument MUST be an array.
But I definitely typoed that code.

Fixed in next profiling branch build. I don't know when/how we can get that into stable.