An example to reproduce this issue can be found in this repository

The issue is easily reproducible with this simple mapping:

@Mapping(target = "nested", source = "mySource")
MyTarget map(MySource mySource);

Where MyTarget.nested is of type MySource

When unmappedSourcePolicy is set to ReportingPolicy.ERROR, the report produces Unmapped source properties error (or warning in the case of ReportingPolicy.WARN)

The mapping code is generated correctly. For ReportingPolicy.ERROR, the following workaround is necessary in order to get a successful build:

@BeanMapping(ignoreUnmappedSourceProperties = { "field1", "field2" })

Where "field1", "field2" are the fields of MySource

MapStruct version is 1.4.2.Final

Best regards
JC