This is most likely normal behavior, not a defective filter. Two things are happening:

1. Mechanical vignetting: the extra thickness from stacked filters and step rings can block the edges of the image, causing dark corners/sides.
2. Variable ND polarization artifacts: variable NDs are two polarizers. On wider fields of view, light hits the filter at different angles, so density becomes uneven. At stronger settings this can create dark side bands or a dark center/"X"-like pattern.

Your 20mm on Micro Four Thirds is less extreme than a true ultra-wide, but the effect can still appear, especially near maximum density and with added rings/filters.

Try this:

• remove the UV/extra filter
• minimize or eliminate step rings if possible
• test the variable ND by itself, close to the lens
• reduce the ND strength
• use a longer focal length if possible
• crop if needed

If the issue disappears when the filter is used alone, the main culprit is the stacked hardware. If uneven darkening still appears at high settings, that’s a common variable-ND limitation.