Sue

Here are more details.

Eph. 5:21

submitting to one another out of reverence for Christ.
1 Clement 38.1:

“So in our case let the whole body be saved in Christ Jesus, and let each man be subject (hypotasso) to his neighbor, to the degree determined by his spiritual gift,”
2 Macc 13.23,

”[King Antiochus Eupator] got word that Philip, who had been left in charge of the government, had revolted in Antioch; he was dismayed, called in the Jews, yielded (hypotasso) and swore to observe all their rights, settled with them and offered sacrifice, honored the sanctuary and showed generosity to the holy place.”

However, Grudem writes,

But in spite of all these different forms of submission, one thing remains constant in every use of the word: it is never “mutual” in its force; it is always one-directional in its reference to submission to an authority.

It clearly is mutual in Clement, and it clearly is not in reference to an authority in 2 Maccabees. I don’t find that Dr. Grudem’s research reflects the broad range of Greek usage or patristic exegesis.