Hello,
I have the same issue than you with service PO with multiple account assigment, I have reported the issue to SAP.
I could fix wrong PO executing report RKANBU01 and it fixed the wrong omiitments.
But I reproduced the issue in new PO in development and I am pending from a SAP answer.
If the report doesn't fix your commitment implement if you don't have impemented these 2 OSS notes (i had to implemented)
680078
1494533
Please share your results!
Regards