Hi,
It seems, you have maintained the 'Mixed Storage in HU' with the value '1' ( Mixed Storage Not Allowed ),
Impact of this indicator is ' A handling unit in a storage bin can only contain one product and one batch (one quant) '.
Could you please check the HUs , either these are with same products and batches.If these are the mixed HUs ( different products 'or' same product with different batches ), system tried to find the Bins to store the relevant ones in one bin as per the putaway rule addition to existing stock.
C K Reddy