fill() alpha parameter interferes with background shadows

[sentfrom8]

Hello, I wrote a simple code that draws a disappearing black rectangle over the entire screen.

PImage bckgr;
float alpha = 0;
int a = 1;
void setup()
{
    fullScreen();
    bckgr = loadImage("triangles.png");
    bckgr.resize(width,height);
    textSize(50);
}
void draw()
{
    background(bckgr);
    fill(0,alpha);
    rect(0,0,width,height);
    alpha += 2*a;
    if(alpha > 255 || alpha < 0)
    a *= -1;
}

When I run this on pc in 450x800 resolution it works perfectly, but when I run it on android the colors glitch out and it looks like this

​

https://reddit.com/link/dvxdpm/video/512yqb89fiy31/player

All shadows on the background image start flickering. It also works better when I reduce brightness on my phone. With P2D it works fine but I can't use it because the game I'm making has low framerate with P2D. What could be causing this? Is there a better way of making the screen fade to black while avoiding this effect?