Probably because you have an error in your program. Or in your SVG file. Since we have neither, we can only play guesses...

I also see that Texas (I recognize it because of the Texas Instruments logo...) has a strange line across it. And perhaps around the Great Lakes. Again, not knowing how you display the SVG, I don't know why they are there.

  cc.beginDraw();
  cc.shapeMode(CENTER);
  for(int i=0;i<states.length;i++){
    
    PShape state = usa.getChild(states[i]);
   
    state.disableStyle();  
    cc.fill(i*5,i*5,50);
   
    cc.shape(state,620,370);

  }
  cc.endDraw();

  image(cc,0,0);


Here is where I create the Pgraphics to display the SVG and color the states.

Thanks!!

Also, this only happens when I draw the states in the buffer.  If I draw and fill the state outside the buffer, the state draws properly and fills properly. 

I duplicated the relevant part of your code, using a USA map found on Wikipedia:

1. PGraphics buffer;
2. PShape usa;
3.  
4. void setup()
5. {
6.   size(1000, 650);
7.   buffer = createGraphics(width, height, JAVA2D);
8.   usa = loadShape("usa-wikipedia.svg");
9.  
10.   buffer.beginDraw();
11.   buffer.smooth();
12.   buffer.background(255);
13.   buffer.shape(usa, -250, -150);
14.   int count = 0;
15.   for (PShape state : usa.getChildren())
16.   {
17.     state.disableStyle();
18.     buffer.fill(count * 5, count * 5, 150);
19.     buffer.shape(state, -250, -150);
20.     count++;
21.   }
22.   buffer.endDraw();
23.  
24.   image(buffer, 0, 0);
25. }

I don't see the artifacts you show. Perhaps you have a different SVG file.

Thank you for your response.  Because I couldn't figure this problem out previously, I wrote code to display the states with different colors (not in a buffer) and then just drew the full map over them.  This seems to work fine, but do you know if there is any reason that drawing in a buffer would be better than drawing them under the other map?

No, because as I wrote, I cannot reproduce your problem. Drawing on the screen is actually drawing on the main PGraphics of the sketch, so these operations should be similar, or identical. Unless you use a different PGraphics, like JAVA2D for one, P2D for the other.

I just tried my code above, changing JAVA2D to P2D, and I got the same rendering than you...