After playing with settings a lot seems that setting fanmin 30, fanmax 100, tt 50 did the trick. Still not obvious and unpredictable though. At least fixed fan speed still not working.
This is for ROG cards with 0db function. Non-ROG Strix RX470, Strix RX570, RX480 work as expected.
Update: trick did not work. Maybe I'm doing something wrong? I want to keep coolers running at minimal set speed (30% for example) even if GPU is colder than -tt specified. How should I set it up?
To be honest - I tried claymore and it handles these cards even worse - coolers always start running at 100% when -tt condition is matched.