OSS note 399906 - Valuation with deviating purchase order price unit explains the valuation logic.
And as always with moving average price, the value of the stock gets adjusted if the invoice value is different. The already done goods issues cannot be re-valuated, hence you get bigger deviations of the price fo your material and the remaining quantity can get far apart from the PO price.
But there is not much to do except you urge your vendor to send the invoice timely, e.g. by EDI.
Or you block the stock until the invoice is received, which is not good practice. Or you change your valuation logic to standard price and post all differences to a difference account