Why are my swagger docs showing 'additionalProperties = false' for my custom schema filter?

Question

I have this SchemaFilter in my swagger config

public class SmartEnumSchemaFilter : ISchemaFilter
{
    public void Apply(OpenApiSchema schema, SchemaFilterContext context)
    {
        if (!TryGetSmartEnumValues(context.Type, out var values))
        {
            return;
        }

        var openApiInts = new OpenApiArray();
        openApiInts.AddRange(values.Select(x => new OpenApiInteger(x.Value)));

        schema.Type = "integer";
        schema.Enum = openApiInts;
        schema.Properties = null;
        schema.Description = string.Join(", ", values.Select(v => $"{v.Value}: {v.Name}"));
    }
}

It is working well, but for some reason this "additional properties" always appears in the docs:

But only for this particular schema - all the other schemas don't have it. Is there some way of removing it?

I tried setting both

schema.AdditionalProperties = null;
schema.AdditionalPropertiesAllowed = false;

but it made no difference