Rendering when the watch face is in ambient mode does not show anything on the screen, for example this sketch:

void setup() {
  fullScreen();
  strokeCap(ROUND);
  stroke(255);
  noFill();
}

void draw() {
  background(0);  
  if (wearAmbient()) strokeWeight(1);
  else strokeWeight(10);
  float angle = map(millis() % 60000, 0, 60000, 0, TWO_PI);
  arc(width/2, height/2, width/2, width/2, 0, angle); 
}

should show an arc with a stroke weight of 1, but the screen is black.

Tested on a Samsung Galaxy Watch 4 with Wear OS version 3.5. However, other non-Processing watch faces also display a blank screen when the watch switches to ambient mode... maybe it's the default behavior of this particular device. Some discussions that could be relevant:

https://www.reddit.com/r/GalaxyWatch/comments/pjgdl6/ambient_mode_kicks_in_prematurely_and_blocks/

Setting Always On Display to ON does not seem to solve the issue.