MIRO changing the vendor as it is "posted"

[MPM124]

Hello All, 

I have a unique issue where the vendor of an invoice is being changed upon hitting "Post" in MIRO. I have not identified any consistency with the invoices or the vendors that are being changed, and no Partner function exists that would be causing the invoicing party to change. Our users also insist that they are not manually changing the vendor using the "Different Invoicing Party" function as well. I have reviewed SAP notes such as Note 393431 but they don't appear to explain the problem we're experiencing.

My question is, how does MIRO determine that the vendor of the invoice is the correct vendor? I have read that MM Invoice Verification matches the invoice's vendor to the PO, but in my case, it switches the vendor that is seen on the PO to a seemingly random vendor when posting the invoice. I have also been unable to replicate this issue, as when I attempt to do so in our testing environment, I consistently pull the correct vendor. It's relatively uncommon for our users as well, but a few times every week this will occur, and the incorrect vendor will get invoiced.

I would appreciate any insight on how to determine where the transaction MIRO is pulling these incorrect vendors from. 

Thank you,

MPM124

[Vijay_SAP]

Hi @MPM124 
In MIRO transaction by default vendor copies from purchase order. If any partner function (payment vendor) exists, then that particular vendor automatically copies to MIRO screen.
Also If po has freight condition types (which are linked to freight vendors), then while posting Invoice for freight conditions, that particular vendors gets copied to MIRO screen.
Above are the standard SAP behavior.
If the Issue is happening for only particular user. we may modify the warning message to Error for M8 - 286 message for that user only. (SPRO > MM > LIV > Define attr. syst. msgs)
If Invoicing vendor changing functionality not using by your client, you may put the error message without any user name in the above configuration path.