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So the mob's normal social assist still applies - there are just some line of sight checks going on when it's a sneak pull. It checks if the mob you tag is looking at you, and if that fails (meaning it's looking away and therefore a potentially successful sneak pull) then it checks for mobs within its normal social aggro radius. If mobs exist = true, then it performs another line of sight check between those mobs and you.
However, here's another potential mechanic, which would partially explain what you're describing:
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Even in that scenario, there would still be a line of sight check between you and the mob you're tagging, and if that check = true then it would use its normal social assist - the sneak tag would fail. In other words, the other mob that's looking at you would only stay put if the mob you're tagging is looking away.