Most common cause of this problem is the barrel's bearing hard at one place around its shoulder against the receiver face. As the barrel heats up and expands, more pressure's applied at that point and a stress line goes down the barrel bending it.
Sometimes, even with the receiver face squared up but the barrel's not homogenous all over in its steel properties. As it heats up, the metal expands more on one side than the other and the barrel bows away in that direction.
In either instance the muzzle axis starts pointing some place other than where it did when the barrel was cold.
I'd sent that rifle back to the maker and if its in warranty, they should replace it.
All this assumes there's no external pressure being applied in increasing amounts by the shooter through the stock's fore end that touches the barrel and bends it a tiny amount. Such things often happen when someone claims a dollar bill slides easily between the barrel and fore end. A dollar bill's .0045" thick. It doesn't take much pressure on the fore end to bend it at least that much and make it touch the barrel. Most often this is the cause when resting the fore end atop something on a bench top and the shooter bears down on the stock's cheek piece with varying amounts of pressure.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master