I would be plugging it into VCDS to see what fault codes are flagged.
Checked for leaks/ along the system?
If could be a cracked Fuel Pressure Regulator Valve (N276)........the big one above the HPFP as they can crack in half. Usually caused by people pressing too hard on the engine cover in that area, and the connector side applies too much levering force and then CRACK!
Even a hairline crack would mess things up and superglue can be used as a temp fix.