For a note block, if the base block underneath is pushed by a piston, no matter whether the new base block is the same as the old one, the note block's instrument will unconditionally changes to "harp" for a short period, because the base block is replaced by "moving piston" during piston pushing.

This behavior prevents the observer from telling the player whether the instrument (the base block's type) has changed, because observer will unconditionally emit a pulse for the temporary wrong "harp" instrument.

Therefore, personally I think the note block should not respond to state update derived from "moving piston", so note block's instrument type will be consistent if its base block type is consistent.