The command dfilt.df1 creates an object (a "discrete-time, direct-form I filter object".) That object has a property named PersistentMemory, and from that documentation page that property "Specifies whether to reset the filter states and memory before each filtering operation. Lets you decide whether your filter retains states from previous filtering runs. False is the default setting."
You can't get rid of it, it is part of the object.
