Each injectors unique flow rate information has to be programmed into the ECM, or codes/CEL will result. If they replaced 7 injectors, then this process was also done. You will want to look for a record of those flow rates in the reprogram step.
If there is no record of the TSB service performed, then get the TSB and review exactly what service is performed, bring the TSB to a dealer with your truck, talk to someone who has performed the TSB on other trucks, and ask him if it has been done to yours. If it has, you pretty much have your first legal witness.
The description of this problem given by the OP, does not describe a systemic fuel injector failure. You can't "reset" damaged fuel injectors. The symptoms of bad fuel injectors are very much visual and full time power impacting.
The first thing you do with a P0087 is change the FF, a routine step regardless of other theories. And save it for future dissection depending on the outcome.
What YOU very likely have experienced is the aging fuel line collapse under suction. The lines lose stiffness with age and heat. Your July timeframe reinforces this, summer is when it will show up first, it can even disappear for the entire winter. It became a common issue with your generation of duramax, with an easy fix. Unfortunately it will usually not be something that can be duplicated in a dealer's bay, after the fuel lines cool down (and become stiffer).
Non-warranty work for stranded motorists is an unscrupulous dealer's dream come true. Any truck that intermittently shows symptoms, is always a sensor or hard part anomoly, not a systemic issue like fuel injectors.
There is a TECH BULLETIN on this specific issue, and if it was never brought to your attention, and eliminated as a cause, then you have proveable legal recourse.