With a 1-8" twist you should be quite able to stabilize and get good groups from the bullets you state. Any keyholing in the bullet holes?
One thing I know is when you get data for the 6-.284 it's below what the cartridge will really do. Way below. A website I use max's me out at 48 gr. I'm really at minimum decent operating pressure at 51 gr. I have plenty of room to go up from there, but per the website, I'm over their max. It's a liability thing. So, you've got to decide if you want to take that wildcat up higher. Keep an eye on ANY pressure sign along the way.
That said, I don't think it's your barrel twist. You might have way too low pressure in the load or who ever put your barrel on didn't get something right. What are your primers looking like. Any issues with the case head not sliding back into the shellholder? The .284" cases are known for that.
FWIW, my load is 51 gr. of RE-19 (you need a slow powder) behind a 100 gr. bullet. The shape of Berger and new Sierra 95 gr. bullets dictate a tighter twist than what I have, 1-10".