It's a pillar and glass bedded laminate stock with very sufficient clearance between the barrel and the stock. The world is full of guns that work just fine with this setting so I'm having a hard time to think guns couldn't work with pillar bedded laminate stocks. Also worth mentioning it has a varmint barrel.
12 twist for 223 has been around for ages and is widely used. It sure does limit the length/weight of the bullets to be used, but there are millions of guns that shoot just fine with the 12 twist. Also at least brownells states the 53 grain vmax should work with 12 twist. Also the stability should be better in hotter climate, not worse. On the other hand, when things were working it was a 100 m range, this time it was 150 meters. The 53 grain vmax is a long bullet for its weight and the stability depends on the length of the bullet, not the weight, so some heavier bullets may actually be even shorter if they are blunt nose and flat base.
The zero was just fine, it's the grouping that sucks. Or well, difficult to say if it's half an inch this or that way when the group is all over the place, but no significant zero change.
Thank you all for the comments. I still think the two best guesses are
1)Something weird with the stability, maybe starting to lose the stability after 100 meters. I know the stability of the bullet with 12 twist could be better but as mentioned, it has grouped just fine in colder weather
2)The barrel harmonics node is super narrow which causes the load to "exit the node" when the temperature changes.
The poor grouping was consistent and repeatable while I was shooting way better with other loads.
I'll try the freezer trick next time I go to the range and report back. There's a risk we might learn something from this