My PX4 Copter has problem with uploading missions

Hi, I have a question about PX4 mission uploading procedure.

As the MAVLINK mission protocol says when uploading missions to drone, GCS should send MISSION_COUNT first and then FC(drone) sends MISSION_REQUEST_INT to GCS.
After that for each MISSION_REQUEST_INT sent by FC, GCS has to send MISSION_ITEM_INT to upload the missions.

Here’s the odd situation. MISSION_REQUEST_INT has some properties such as missionType, seq, etc…
Isn’t the seq of MISSION_REQUEST_INT supposed to be increasing when GCS sends MISSION_ITEM_INT to the drone?
My FC keeps sending MISSION_REQUEST_INT with seq 0(not increasing at all).

Anyone knows about this kind of situation?