I don't know the EMB-190 QRH, but I was involved in the evolution of the Boeing fuel leak procedures over the last 30 years, and the system architecture is effectively the same with respect to this issue. The reason they want you to stop any crossfeeding in progress is to allow you to determine whether the leak is in the left or right engine fuel system. (Large leaks are almost always from the plumbing, not the tank itself.) Once you have verified a leak on the left or right side by comparing fuel flows and watching FQIS dropping over a period of time, the procedure (at least on the Boeing airplanes) is to shut down the engine on the affected side to stop the fuel loss primarily to preserve range, and also to reduce the risk of fire. You then crossfeed as needed the same way you would for any other engine shutdown. Depending on the mission, the fuel leak rate, and where you are in the mission, you may need all of the fuel in both mains to make your destination, alternate, or diversion airport on your remaining engine.
So, Nomad2, you were correct that you don't just keep the crossfeed closed, but the second instructor probably expected you to shut down the engine on the affected side, which I suspect is the QRH procedure. Keeping the leaking engine going and balancing into the leak is what led to the A330 dead-sticking into the Azores. This link tells that story if you aren't aware of the event.
https://lessonslearned.faa.gov/ll_ma...=73&LLTypeID=2