Simulink set_param from callback doesn't take effect until after simulation

I have a Simulink model that I am trying to test with unittest and sim(), and need to update several const block's values during a test. To do this, I added a clock to the diagram and attached an event listener to it:

add_exec_event_listener('myModel/Sim Time', 'PostOutputs', UTCallback);

That event listener (UTCallback) should update the value of a const block based on the clock's time (note: it also does several other things which is why this functionality isn't in the simulink diagram). The const block I'm trying to update uses a variable for its value (myVar). The problem is, that update doesn't take effect until after the simulation has run. I inspected the simulation graphs and the value doesn't change during the simulation. Immediately after the unit test completes though, the block in Simulink reflects the change, and the next time I run the unit test the new value is used (but another change during the second run isn't reflected until the third run, etc).

Here is UTCallback:

function UTCallback(block, eventdata)
    simTime = block.OutputPort(1).Data;

      if(simTime == 10)
          updateConst(1234);
      end
  end

Here are two updateConst functions I have tried:

function updateConst(value)
    assignin('base', myVar', value); 

      set_param('myModel', 'SimulationCommand', 'update')
  end

function updateConst(value)
    set_param('myModel/constblock',...
       'value', '200');
    set_param('myModel', 'SimulationCommand', 'update')
end

My const block has the following settings:

• Const Value: myVar
• Interpret vector parameters as 1-D: unchecked
• Sampling mode: Sample Based
• Sample Time: -1

UPDATE: I discovered the following which might be related. If I put this code inside updateConst:

disp(get_param('myModel','SimulationStatus'));
set_param('myModel', 'SimulationCommand', 'pause');
disp(get_param('myModel','SimulationStatus'));

It prints the following:

running
running

Regards, Istvan.