Turbo charger seizure

The turbo charger in my car recently started making a whining noise when the car was accelerating. I put the car into the dealership because they replaced the turbo charger approximately 2 months ago after the previous one seized and the turbo shaft snapped. They also replaced the inlet pipe, seal, oil filter and performed an oil change. The new parts came with a warranty. Now that the new turbo is whining (the car has done approximately 5000 miles since the turbo was replaced) they have tested it and initially believed there was an air leak on the inlet side. They have now revised this idea and are now stating that there is low oil pressure in the engine which is starving the turbo of oil. As part of the replacement of the turbo last time, would you have expected the garage to perform an oil pressure test and could the replacement parts (e.g. new filter or inlet pipe) have contributed to the low pressure issue now?
Whenever a turbo is replaced the oil feed pipe to the turbo bearing needs to be replaced and the oil return pipe from the turbo bearing back to the block needs to be replaced because these pipes typically get blocked by carbonised oil from the heat of the turbo. However, if the engine is additionally suffering low oil pressure because the sump strainer is blocked, because the oil pump is worn, or because of wear in the crankshaft, then that is another issue.
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