Standard approach would be to have a visable piece of fuel line in the injection pumps output. To verify no air content. No air? What fuel pressure is present in the base of the injection pump should be next.

Also any time any form of fuel supply issue potential is identified. A change of fuel filters is good practice. If they make no difference put into fuel proof containers and kept in the trunk. For emergency spares is not a bad practice. Plus the tools to change them. You can bypass that if you test for fuel pressure and have adequate.

In your case especially if there is no air present in the output of the injection pump filters should be checked . Too many times the problem just being dirty fuel filters resulting in a long drawn out search for the issue has occurred. Sounds like you just have the all too typical issues of air entering into the system or a fuel starvation issue.

Initially I would check their fuel line work for any partially collapsed or typically kinked lines. They messed up something somehow. Or if they diagnosed the problem perhaps got it wrong. You can almost always rule out coincidence in these situations.

My guess is they did not even do the preliminary tests to identify the real problem. When you took the car back. Injection pumps will not function normally with incoming air in the fuel present . Or the fuel is supplied at inadequate pressure. This indicates the volume is possibly substandard as well.

A systematic approach is far better than just guessing. Why did you originally take the car in for fuel line work? What was the issue at that point. There are some questions in my mind as normally you do not use a garage without an issue being present. If it was low power before you took it in is going to change some things. Or perhaps not depending what it was..