hmm good question.
probably a bug/problem with P2D.
is there a reason why you choose P2D over the detauls JAVA2D though ?
P2D is a bit faster but not as accurate. So I guess in your case just removing P2D solves the problem.

I have switched to JAVA2D, but the bug is still there.

this code where i just removed the P2D runs fine here...
isnt that the way it should look like :

1. void setup () {
     size (600,400);
     smooth ();
    
     strokeWeight(2);
   }
   
   void draw () {
     background (255);
    
     fill (0);
     stroke (255,0,0);
     ellipse (150,200,200,200);
    
     fill (255,0,0);
     stroke (0);
     rect (350,100,200,200);
   }
   

Yes, that's it. I switched and it works, actually I mean with "the bug is still there" : nothing had been debugged, we have just avoided the problem. However it's good to know where are located the bugs.