I've been asking this already directly on TP-link email support where person from technical support team (Daria Hong) tried to answer it, however I am now even more confused from those answers as each new reply contradicts the previous one and none of them are directly addressing my simple question.

my question: is it a bug in firmware or is it like that by design, that all my wireless networks (main Wi-Fi network, guest network and IoT network) sit on the same channel ?

in one reply, I was told that "the router will auto adjust channels and balance three networks", but this is not the case. they are always on the same channel even if I run "Network Optimization" from Tether app