Yeah, probably right. But I can swear it felt like cog slipping and lock-tite to the cog didn't solve the problem.

Have we rulled out the possibility that the cog is threading on too far, such that the lockring is bottoming out on its shoulder before it presses against the cog? This can happen with cheap cogs that are not wide enough, and a spacer is needed between the cog and the hub to move it out far enough to contact the lockring.

+1
certainly a possibility that should be addressed when the OP disassembles to inspect for thread damage