I've noticed this before. I think it would already be fixed if it was actually a bug, though.
This website is for bugs only, but I'll tell you one thing.
It's almost impossible to fix, you might need an NBT editor a world editor. That might make your world even worse, if you aren't careful.
I'm pretty sure you need all source blocks.
Well, I couldn't find the iron ore again. But I did find some even stranger generation.
[media][media]
Okay, I'm adding it right now.
I accidentally opened it in 1.12.2. I should have read the warning.
It seems to work in the new one I built. Maybe it was fixed in 18w08b? I'm not sure, but I'll add an image of the other one.
edit : The old one still doesn't work. It might have something to do with a missing water source block, but I'm still not sure.
Unfortunately, I have lost the coordinates.
I'm almost certain that the seed is 9210560948155018615. I will also edit the description to add this.
Which version is this in? 1.7 or the new snapshots?
This is probably just something with your computer, as I have never noticed this.