Ive hear some boat tailed bullets spiral a bit at closer ranges and stabilize and decrease the spiraling effect at a further distance. I have no idea if its true but have heard bench rest and long range people talk about it.
The bullet can loses stability, yes. But it is at the trans sonic range, where the bullet drops below the speed of sound. Instead of a spiral that the bullet would drill a hole, it begins to yaw causing it to draw a circle. For a 6.5 Creedmoor with a 140 gr A-max, a Density Altitude of 3000', and temps in the 90's, that is around 1400-1500 yards.
I'd have to run the numbers for your Grendel, but it will be give or take 1000 yards.