Have you check the following?
Is the wheel out of dish? If so you could be putting the wheel in straight by eye using the horizontal dropouts, then when you put some power through the wheel it shifts in the dropout so the hub sits straight and the out of dish wheels causes it to be of to the side?
Is the frame straight, just because it's new / not that old doesn't mean it'll be straight. Judging by the quality of most trials frames this wouldn't surprise me.
Are the brake mounts welded on straight / square?
Would be worth trying a different wheel and see if the same issues arise, this will help to identify the issue.