There is a good scientific reason for the "dummy" glass filter requirement in this particular EF EOS R adapter:
The physical thickness of the mount adapter for drop in filters is slightly different then the base EF-EOS R or EF-EOS R with Control Ring. That difference is because the "Drop-in" Adapter is always designed to have a thickness of optical glass in the light path. Optical glass typically has a refractive index of between 1.5 and 1.67. which optically reduces the effective thickness of the optical path-length of the adapter.
With the glass filter in place (doesn't matter if it's a ND, a polorizer or simply clear) the lens flange to focal plane distance is reduced. Ideal lens focus will be at a different position of the RF internal focus positioner. That may make no difference for relatively close subjects, but may make perfect infinity focus poor for some EF lenses.
Ideally, not having an extra layer of glass with it's two partially reflective surfaces would be preferred when no filter is needed (like the other EF-EOS R adapters), but could also create the possible focus limit because the adapter is designed to always have the glass layer in place for proper infinity focus.
Your choice is a $100+ "dummy" glass "filter" or 1/10 of a cent's worth of electrical tape and a hope the that auto focus range of your particular lens will be able to fully compensate. Older, always mechanically-coupled EF lenses may not have enough range to work correctly.
Alternately, for the same or less cost than a drop in piece of clear glass, you can purchase the basic, no ring Canon EF-EOS R Adpater, which is the correct choice for when you don't need a drop in filter.
