ExecuteSqlCommand with interpolated string results in 'invalid input syntax' error

According to this docs and this, I should be able to pass an interpolated string to ExecuteSqlCommandAsync like this:

public async Task DeleteEntries(DateTimeOffset loggedOn) {
    await myContext.Database.ExecuteSqlCommandAsync(
        $"DELETE from log_entry WHERE logged_on < '{loggedOn}';"
    );
}

However, it gives me the following error: Npgsql.PostgresException: '22007: invalid input syntax for type timestamp with time zone: "@p0"'

1. loggedOn is a valid date.
2. Extracting the interpolated string to a temporary variable fixes it, however I am losing input validation for SQL injection attacks.

Am i doing something wrong or is this a bug in EFCore? Using latest EFCore 2.1