I discovered this happens for many other fields - on different libraries. It looks when you create Target Content Types attached to libraries / lists you use Field Display Name instead of Field Id (field Display Name is not unique / Id is). It also seems you always look for fields in Site Columns - but there can be List Columns also (specific to one list only).
Please try to resolve this ASAP as this is crucial issue in our migrations. Change Priority to Severity 1