Short stroke on new build

[A5BLASTER]

I just find it strange that the 6.5 wouldn't have enough gas to lock the bolt back.
I used this lower on my .556 with carbine length gas system with no problems.

Yes but your 556 had a gas length that is standard for it's over all barrel length.

Your grendel barrel has a longer then standard gas length for its length.

I had that same barrel and gas block and had the same issues as well with the jp spring. Now it could be something else but changing out the spring is a cheap and easy thing to test before trying other things.

Changing it out to a standard rate rifle spring fixed the issue.

I have since move on to the jp scs systems for all my ar grendels.