I have lots of beat in putters, but I believe that they all started out way more stable than the flippy fairway drivers do out of the gate. They just don't tend to make putters with -4 turn. I've never had a Sonic though. Plus, with blunter noses to begin with, they don't beat in as much, I think?
I've thrown thousands of field shots and hyzer flip with my putters just doesn't tend to happen, whereas the flippy fairway drivers do it easily if I throw it right. Perhaps dreadlock is correct that I just ending up a little throwing nose up if I'm throwing my slower disc on hyzer. But, if that's the case, I'm finding it easier not to make that mistake with the faster discs.
Just curious, but have you ever actually thrown a 150 class Diamond? They are just stupid easy to get up and gliding. Very flippy, but don't feel like a paper plate like, say, some of the DX Leopard light weigh starter discs will. It fairly quickly became too flippy to use regularly, but it's always a hit for brand new players I take out.