When you plant big spruce (2*2) around it appears a circle of podzol

I'm pretty sure this is intended behaviour.

Added 18w06a.

https://minecraft.gamepedia.com/Podzol

Quoting the minecraft wiki:

"When mega spruce trees grow, they will convert any grass blocks that are nearby to podzol.‌"

so, I'm guessing WAI (Unless you mean that a circle of podzol is appearing out of thin air, which would be a bug)