Clearly that isn’t supposed to happen. Mulch (any mulch…not just the preen product) piledup and around the plant will slowly kill the plant be it a daylily or a Maple tree. Brush the mulch back away from the stem 4″ or so until you see the original soil where the leaves enter the soil and keep it that way. Mulch is good but not when it is piled up around and actually touching and smothering the crown. More on the topic HERE.