This happens now and again. Problems with dealers can arise if a warranty claim is required.
Dealers will almost invariably not pay warranty labour, they always want the vehicle into their workshop. This can be inconvenient.
My experience over 40 yrs is that some dealers, when they get the car into their shop, can 'try it on' ie. the part wasn't installed correctly and so on.
Incidental to that, we have had more problems with VW OE water pumps being of poor quality than some of the factor supplied ones.