Play in the nut should not matter that much. The squared faces of receiver and nut along with the concentricity of the threads of the barrel and receiver are the important parts. A corollary can be made to a shouldered barrel. The barrel threads and shoulder must be square to the receiver threads and face. The taper of the threads will 'center' the barrel in the receiver bore when it is torqued down.
The play in the nut will affect the strength of the threaded portion. Not really an issue for our application.
I would assume (bad thing) that most of these nuts are being made on a CNC. If so it could be as simple as someone setting up the machine wrong, or having an improper cutting bit installed. If the mfg relies on manual QC they might not check carefully enough. Easy enough to mess it up. Sometimes a nice phone call or email can fix the issue.
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