I've seen this multiple time and also on *A partners. On a vacation a few months ago I had my family flying on tickets bought with my miles, while I was using a paid ticket.
Their itinerary had my UA Gold number (specifically my son which does not hold status - came up w my MP number) and even got lounge access based on that.

They must have the MP account number on the reward reservation - to show which account this came from, otherwise it's an issue.

I've had this happen every time I used my miles and vice versa. No exceptions.
You can have the gate agent manually change the MP number for CPU - but only at the airport and only for one flight at a time, since the record mush show the originating MP account.