Mafid "&" is different to valid Mafid "&"

What causes this issue?

The mafid of the first allocation record is compared with the mafid of
other allocation records. If the mafids are different, the record is
ignored.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.