Hello,
I've found that when you use an upgraded version of Fireball, it leaves a hole with fire on some places. While I think that the whole hole is supposed to recover, only there where there is no fire does. So that leaves you with an area with a few holes in it where there was fire from this spell. The same happens with Grenade.
Thanks.

So I'm afraid I can't reproduce this- do you think you could make a video or maybe explain more? Here's what it looks like when I cast Fireball Level 4:

You're right. I had this issue because I was playing with Magic 7.6.6. I updated now and everything works. Thanks!