Try running ionic resources -s to generate splashscreen many times, the task is not complete, some splashscreen not generate successfully.
My splash.png file is about 3Mb (2732px X 2732px), does it have something to do with the file size?
My system info:
global packages:
@ionic/cli-plugin-proxy : 1.3.1
@ionic/cli-utils        : 1.4.0
Cordova CLI             : 7.0.1 
Ionic CLI               : 3.4.0
local packages:
@ionic/app-scripts              : 1.3.12
@ionic/cli-plugin-cordova       : 1.4.0
@ionic/cli-plugin-ionic-angular : 1.3.1
Cordova Platforms               : android 6.2.3 ios 4.4.0
Ionic Framework                 : ionic-angular 3.5.0
System:
Node       : v8.0.0
OS         : OS X El Capitan
Xcode      : Xcode 8.2.1 Build version 8C1002 
ios-deploy : 1.9.1 
ios-sim    : 5.0.13 
npm        : 5.0.0