I'd double check your harmonic output. Might be that you are overloading your spectrum analyzer. Add more attenuation and see if you get the same result to see if that is the case. But the plot you show for the 3.5 MHz harmonic at 7 MHz only shows about -47 dBc rejection of the harmonic. Marker 2 at +15.8 dBm minus marker 5 at-31.6 dBm = 47.4 dB. It is good practice for this to exceed the CFR 47 Part 97.307 with margin. 43 + 10LOG(P) at 400W - 43+10LOG(400) = 69 dB suppression. If you intend to run higher power you need even more suppression.