Why SORTUPBY return wrong value in IBM ITX?

Question

This is the maprule:

=SORTUPBY(FF1(Mission:In1),manufacturerCount:Mission:In1)

syntactically it is ok.

This is the input:

[{"mission_name":"Thaicom","manufacturers":["A"]},{"mission_name":"Thaicom-2","manufacturers":["A","B"]}]

This is the output:

[{"mission_name":""},]

Expected output is:

[{"mission_name":"Thaicom-2","manufacturers":["A","B"]},{"mission_name":"Thaicom","manufacturers":["A"]}]