Looking into booking Air Kenya for an upcoming trip to Masai Mara (Wilson - Mara-Keekorok).

When I go through Air Kenya website directly to try and book, it's not letting me book a return ticket where one leg is on "SPECIAL" fare and the other is on "Y STANDARD". Does anyone know whether this is just a limitation that they impose? Or am I missing something?

Alternatively, I've found I am able to book the mixed fare through Booking.com, but a bit hesitant to book this flight through a OTA vs direct with the airline. What are some thoughts regarding this?