Slower burning powder, so increases the gas port pressure, since powder spike is closer to the gas port, then faster burning powders.
So why the American Eagle 55 runs fine, but the slower powder load is causing the short stroking with the bolt unlocking too soon, to high of bore residual pressure, and too much force is being lost by the bolt trying to pull the pressure welded case out of chamber to start with.

Hence, Just look at the spent case rims, for the bolt unlocking too fast or not, since if bolt is unlocking too fast, with the residual pressure in the bore too high, then ejector is going to bend the rims back on the over pressure welded spent cases to the chamber walls.
As for not enough gas port pressure with loads that do produce 55k, see it all the time, with 40~45gr bullet loads, where even faster burning powder is used, and since the peak burn dwel is even closer to the chamber, by the time  the bullet does pass the gas port to allow it to charge, just not enough residual pressure left behind the bullet to cycle the action correctly as well.  

So if bent spent case rims, then unlock needs to be slowed down for the slower burning powder loads to allow the needed time for the residual pressure in the bore to drop move, via heaver buffer to slow down the bolt unlock more.
Or, adjustable gas block or key, to reduce the pressure dwell that makes it to the B/C gas chamber, to slow the unlock down that way isntead.

Think this, and the slower the powder burn rate, the farther to the right the peak spike gets pushed to the right, and which increases the gas port pressure as well.  The higher the gas port pressure, the faster the bolt unlocks.
Also, do not confuse working pressures, with gas port pressure, since again can have two loads with same working pressures, but the one with the slower powder, will produce higher gas port pressures isntead. And again, lighter than 55 gr bullets with even faster burning powder, can reduce the gas port pressure  to really retard the bolt unlock as well to cause short stroking, all while the loads do have a 55K working over all presssure.


Hence unlike a blow back action, bolt unlock is a timing thing, that has the bolt unlocking at the correct time that the residual bore pressure has drop down enough, so the bolt extractor can pull the case cleanly from the chamber wall cleaning in the first place.  Bolt unlocks too soon, then spent case too high pressure welded for clean pull from the chamber, and rig short strokes.  bolt unlocks too last, then not enough force for it to full stroke as well.

So with the slower powder load, your bolt is just unlocking too fast, and its needs to be slowed down.  Heaver buffer is about the fastest way to solve the problem, since if you do want to shoot ammo with faster burning powder, may be able to come up with a buffer weight that pulls the two off, or worse cases, just have to drop back in the lighter buffer for that ammo instead.

The fact that it is running with faster burning powder loads in the 55 gr range, tells me that its not a gas leak or gas block problem to begin with.