Bug in "Used Margin" computation in backtesting cTrader V 5.0.29

I think there is a bug in the "Used Margin" computation when using ModifyVolume(volume).
To prove the bug I did the following (see code below):

1. Open several positions of EURUSD with 1 Lot one after each other
   - The sum of the quantity is 3 Lots ==> OK
   - Each position “costs” 200,- € Margin, so the sum of “Used Margin” is 600,- ==> OK
   
   
    
2. Open one position EURUSD with 1 Lot 
   - Than do a 1st ModifyVolume(2 Lot)
   - Than do a 2nd ModifyVolume(3 Lot)
   - The sum of the quantity is 3 Lots ==> OK
   - HOWEVER, the sum of “Used Margin” is 1200,- ==> NOT OK
   
   
   
   The row of highering the Used Margin is multiplying the lotsize by the single margin value and adding it to the Used Margin:
   200, 600, 1200, 2000, 3000, 4200, …
   THIS IS WRONG!!!
   
   3. Doing the same thing on an demo or live account gives the same correct result as in 1.
   1st Trade: 200,- Used Margin
   1st ModifyVolume(2Lot): 400,- Used Margin
   2nd ModifyVolume(2Lot): 600,- Used Margin 
   3rd ModifyVolume(2Lot): 800,- Used Margin 
   and so on
   
   Here is the code to test and reproduce this:

using cAlgo.API;
using System.Diagnostics;
namespace cAlgo.Robots
{
  [Robot(AccessRights = AccessRights.FullAccess, AddIndicators = true)]
  public class MarginBug : Robot
  {
     int mVolume = 100000;
     protected override void OnStart()
     {
        Debugger.Launch();
     }
     protected override void OnTick()
     {
        if (mVolume <= 1e6)
        {
           if (0 == Positions.Count)
              ExecuteMarketOrder(TradeType.Buy, "EURUSD", mVolume);
           else
              Positions[0].ModifyVolume(mVolume);
           mVolume += 100000;
        }
     }
     protected override void OnStop()
     {
     }
  }
}