In the official MIPS ISA documents, the nmadd/nmsub means "multiply, add/subtract, then negtive".
In the earlier version, the code of nmsub_s is "Fd.sf = -1 * ((Fs.sf * Ft.sf) - Fr.sf);", but the brackets have been changed now.
So change the sign here.

Compare with the gem5 stable version and MIPS ISA official documents.