From looking at an exploded view of the action parts, The block is only secured by the hammer on firing. If the hammer spring is weak, it can allow the action to open under pressure. The action would close again as the pressure returns to normal. This would happen very quickly, giving the impression that the action remained closed. The use of a reload with the slightest amount of sizing lube on the case would add to the problem. That is why the factory ammo works OK.
Compare the spring strength to your other action and check the cases for sizing lube.