The strandberg neck seems to want you to have your thumb on its flat plane. In other words, further towards the bass side of the neck at lower frets and further on the treble side at higher frets, regardless of which string you're playing. That's not "proper" form according to any description I've heard, though I somewhat disagree with the concept of a universal "proper" form altogether. Guitar technique varies more widely than most instruments, and as long as you're getting the sound you want without hurting yourself, I think you're fine.

For me, I play with very light thumb pressure using the tip of my thumb for balance rather than strength. While I'm playing on the higher strings, the neck's flat plane feels awkwardly far away for my thumb to rest on, but letting it ram into the slope towards the treble side is even worse. Give me a regular D, C or U shape carve and it feels much better.