Beginners don't "try" to spin out of shihonage
I've come across one who does, so there are probably more. Not many try to do it, so it probably is incorrect technique (no shame, we've all been there) but it's possible one uke has tried to do it repeatedly and others have seen and copied it. With proper application of the technique, though, it shouldn't be possible.
It should be possible to test for it if you begin the technique and then stop. If they're doing it on purpose, they'll spin while you've stopped, and you can politely ask them what they're playing at. If they stop when you do, then it's your technique driving their spin.