The photo shows a 6 month old post-filter. The clean strips align with the air that passes closest to the ionizing wires.
Inspected up close, the dirty areas are a deep stain. Vacuuming would clean the surface lint but the fine E1 particulate becomes infused like a smoke stain. Honeywell notes the postfilter will loose its static properties if washed.
Inspect your old postfilter. It will begin to collect a layer of dust or lint if cell cleaning is overdue. The postfilter will also show problems such as a missing ionizing wire.