Spotted highlight appears when 'Radiance Size' is high in the environment

[mysticfall]

Godot version:
master / e453934

OS/device including version:
Manjaro Linux 18.0.4 Nvidia GTX 970

Issue description:
I created a scene with a GI probe and a world environment, and I found that unnatural highlighted spots appear if I set 'Radiance Size' in the environment resource higher than 64.

This is how it looks when it's set to 64 (appears to be normal):

And with 128,

And with 256,

I'm using an HDRI image from this website, and it contains quite bright Sun as it represents an outdoor scene at noon: