I see high radiated harmonics with Si4x6x radio chips with even good conducted results. What can be the main source of the harmonic radiation?
If the conducted spectrum looks good in terms of output power at the fundamental and at the harmonics too the harmonic content of radiation is likely not radiated via the antenna. In that case the VDD lines can be suspicious, since the harmonic content (generated in the switched mode PA) can go through the choke inductor (through its parasitic capacitance at the higher frequencies) and can be radiated out by the VDD traces. In that case the harmonic filtering of those VDD traces becomes more important and suggested.
Moreover, traces connected to the SDN, GPIO 2 and 3 pins might be the main radiators of the unwanted harmonics, so the additional filtering of those traces is also recommended.