Why is the image shown incompletely?

Question

I used GEE to load remote-sense image to get NDVI, then clipped and bound it on region. There is not a single problem and the image is complete before I try to load other image on the region.

Why I load NDVI image to the region that will show incomplete?

var roi=ee.FeatureCollection("users/wwhq3d3/alar");
Map.centerObject(roi,9);
var iterator=function(image){
  var ndvi=image.normalizedDifference(["B5","B4"]).rename("NDVI");
  return image.addBands(ndvi)
}
var redu=function(image){
  var dict=image.reduceRegion({
                 reducer: ee.Reducer.mean(),  
                 geometry: roi,  
                 scale: 30 
  });
  var ndvi=ee.Number(dict.get("NDVI"));
  image=image.set("ndvi",ndvi);
  return image;
};
var image = ee.ImageCollection('LANDSAT/LC08/C01/T1_TOA')
                  .filterBounds(roi).filterDate('2019-05-01', '2019-05-31')
                  .map(iterator).select("NDVI").map(redu);
print(image.first());
var visParams = {bands: [ 'B5', 'B4'],min: 0,max: 3000,gamma: 1.4,};
Map.addLayer(image.first().clip(roi),{},"image")
Map.addLayer(roi,{color:"red"},"roi")

enter image description here

I need know how to solve this problem.