After read your reply, I did another 2 tests. I have met the same problem. All h264 streams have different content, but all them were encoded by media sdk encoder (sample_encoder.exe) from raw rgb4 data of different resources.
I believe the older driver you used had some issues with DecodeHeader that were resolved. The expected behavior of DecodeHeader is to parse the stream for sequence header info (SPS/PPS) until following NAL delimiter is found, such as start of I-frame.
For the case when bitstream buffer just contains SPS and PPS but you still need to parse the header, you will have to pad the buffer at the end with I-frame NAL header to ensure successful DecodeHeader operation.