const blip = mp.blips.new(5, new mp.Vector3(0, 0, 0), {
  alpha: 175,
  color,
  name: 'zone',
  dimension: 0,
  scale: 1,
  shortRange: true
});

setTimeout(() => {
  blip.setAlpha(150); // it works

  setTimeout(() => {
    blip.radius = 50; // reseted alpha to 255
    blip.setAlpha(150); // not works
  }, 5000);
}, 5000);