When creating an empty DataSet with unspecified transfer syntax then the transfer syntax is set to 1.2.840.10008.1.2.1

Setting the transfer syntax on a DataSet with unspecified transfer syntax causes the Dicom codec to misbehave when a DataSet is embedded as sequence item into another DataSet.

The explicit transfer syntax causes the code to switch to the transfer syntax indicated by the sequence item, invalidating the entire dicom file.