I guess the command you are missing is:
just check this URL to read more about it
checking the "ArubaInstant 8.x Command-Line Interface reference guide":
there you do find no such restriction - regarding platforms "all" are mentioned for the "preferred-uplink".
Thus I do suggest to open a TAC-Case (if they can't provide a fix for the issue itself they should replace the AP as Workaround)
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