Yes. In your code... Which we can't guess...
If it is too long, try reproducing your issue in a simpler sketch. Sometime, doing that, the problem just shows itself!

Neither me.
Even by supplying the missing code (shown below), I see little graphics, and I have no idea of what is wrong for you.
Code:

int OUTPUT_WINDOW_POSITION_X = 50, OUTPUT_WINDOW_POSITION_Y = 50;

Text1 t;
PGraphics outputScreen;
int sliderValue1 = 21;

void setup()
{
  size(500, 500);
  t = new Text1();
  outputScreen = createGraphics(200, 200, P2D);
}

void draw()
{
  background(250);
  t.generateFrame();
}
 

Ah, "once"! It is supposed to draw several rects? Then perhaps you should put some code in the move() routine, currently you draw all rects at the same place.

In this case, it works perfectly for me. I see a small black square and a little diagonal line beside it.

Classical error, you forgot that draw(), as well as beginDraw(), doesn't clear the graphics buffer (main or specified).
Somehow, the anti-aliasing of borders (graphics and rect, I suppose), when cumulated, erases the drawing.

Notice I didn't had an issue in my test code because I have a background() instruction at the start of the draw.

If I change the code to:
Code:

nt OUTPUT_WINDOW_WIDTH = 8;
int OUTPUT_WINDOW_HEIGHT = 8;
int OUTPUT_WINDOW_POSITION_X = 20;
int OUTPUT_WINDOW_POSITION_Y= 20;


int OUTPUT_WINDOW_WIDTH2 = 20;
int OUTPUT_WINDOW_HEIGHT2 = 20;
int OUTPUT_WINDOW_POSITION_X2 = 50;
int OUTPUT_WINDOW_POSITION_Y2= 20;

PGraphics outputScreen;
PGraphics outputScreen2;

void setup() {
  size(500, 500);
  frameRate(5);
  outputScreen = createGraphics(OUTPUT_WINDOW_WIDTH, OUTPUT_WINDOW_HEIGHT, P3D);
  outputScreen2 = createGraphics(OUTPUT_WINDOW_WIDTH2, OUTPUT_WINDOW_HEIGHT2, P3D);

  outputScreen.beginDraw();
  outputScreen.background(#dce6ee);  
  outputScreen.rect(1,1,2,2);
  outputScreen.endDraw();

  outputScreen2.beginDraw();
  outputScreen2.background(#dce6ee);  
  outputScreen2.rect(1,1,2,2);
  outputScreen2.endDraw();
}

void draw(){
  background(200);
  image(outputScreen, OUTPUT_WINDOW_POSITION_X, OUTPUT_WINDOW_POSITION_Y);
  image(outputScreen2, OUTPUT_WINDOW_POSITION_X2, OUTPUT_WINDOW_POSITION_Y2);	
}
 

(Reminder: keep size() call first of setup() and use numerical values)

Yes, because the graphics don't change...
But indeed, I see now the issue, which is a bit strange. So maybe there is a bug, or a strange side effect at least...
Code:

int OUTPUT_WINDOW_WIDTH = 10;
int OUTPUT_WINDOW_HEIGHT = 10;
int OUTPUT_WINDOW_POSITION_X = 20;
int OUTPUT_WINDOW_POSITION_Y= 20;
 
 
int OUTPUT_WINDOW_WIDTH2 = 20;
int OUTPUT_WINDOW_HEIGHT2 = 20;
int OUTPUT_WINDOW_POSITION_X2 = 50;
int OUTPUT_WINDOW_POSITION_Y2= 20;
 
PGraphics outputScreen;
PGraphics outputScreen2;
 
void setup() {
  size(500, 500);
  frameRate(1);
  outputScreen = createGraphics(OUTPUT_WINDOW_WIDTH, OUTPUT_WINDOW_HEIGHT, P3D);
  outputScreen2 = createGraphics(OUTPUT_WINDOW_WIDTH2, OUTPUT_WINDOW_HEIGHT2, P3D);
}
 
void draw(){
  background(200);

  outputScreen.beginDraw();
  outputScreen.background(#dce6ee);
  outputScreen.fill(0);
  outputScreen.noStroke();
  outputScreen.rect(1,1,2,2);
  outputScreen.endDraw();
  image(outputScreen, OUTPUT_WINDOW_POSITION_X, OUTPUT_WINDOW_POSITION_Y);
 
  outputScreen2.beginDraw();
  outputScreen2.background(#dce6ee);  
  outputScreen2.fill(0);
  outputScreen2.noStroke();
  outputScreen2.rect(1,1,2,2);
  outputScreen2.endDraw();
  image(outputScreen2, OUTPUT_WINDOW_POSITION_X2, OUTPUT_WINDOW_POSITION_Y2);  
} 
 

The display is fine, but if I reduce the first two values to 9 or below, the drawing disappears there.
P2D doesn't have this issue.