1554.md

When an interface method defines an optional parameter, its default value is discarded during overload resolution unless you call the concrete class through the interface reference.

When a concrete implementation of an interface method sets a default argument for a parameter, the default value is discarded during overload resolution if you call the concrete class through the interface reference.

See the series on optional argument corner cases by Eric Lippert (part one, two, three, four) for more details.