Cummins ISB aerated fuel during injector return flow test

Fighting an issue with an ISB where fuel rail pressure falls on its face to around 2k when ECM is demanding 20k+, this only happens at high rpms, sitting still or driving. It has been modified and flashed. It throws no codes and passes all low pressure system tests (supply pressure to high pressure pump, inlet restriction and no air in fuel). CP3 high pressure pump return flow within spec, rail relief valve replaced due to leakage. Fuel pump actuator was replaced due to internal leakage to electrical connector. Customer replaced fuel rail pressure sensor before bringing it in.

While measuring injector return flow volume I start the high pressure system leakage test in Insite and it takes about a minute and the rail pressure gets to 19k while demanded pressure is 24k, the fuel coming from the return rail in the head becomes aerated. I stop the test in Insite and demanded pressure drops to 14k but actual pressure stays at 19k. I rev the engine and aeration becomes severe and rail pressure raises to 24k and stays there. I turn the engine off, start it and rail pressure is back to 14k like the ECM is demanding and it falls down to 2k when rpms are increased, theres no more aeration in the return fuel from the injectors.

Any ideas? this one is throwing me for a loop, especially the fact that its been modified and it throws no codes for low fuel pressure even if you drive it for 5 minutes with rail pressure at 2k while the ECM is demanding 20k.