Hi Sachin,i had the exact same issue as you described (using my own login-form (not Strataframe Login-Form)).
I was finally able to resolve that by calling:
this.Owner.Activate();
within the Login-Forms OnLoad Method.
Make sure you set MyLoginForm.Owner = this; within the MDI-Parent.
Still not sure what causes the application do loose focus, but the workaround above solves it at least. Btw, that was the only Child-Form in my Application with that issue.