has the same issue,

to solve it you need to the Item Receipt and Delete the receipt of item that the average cost start to change.

( you don't need to delete the PO)

the simplest way to do it is going to the purchase order and clicking CTRL+H it will show the item receipt 

and delete that item in the receipt or delete the entire receipt.

... and then save everything should balance.

Don't forget to create a new item receipt.