Hard to say what they may have done to cause this issue.

I would start by scanning for trouble codes and then doing a quick volume test on the low-pressure side at the CP3 pump.

This will give you an idea of where to start and let you know the lift pump and filters supply enough fuel to the high-pressure pump.

They may not have torqued the injector crossover tubes correctly; a fuel return flow test would likely be next on my list.