Yurii,
I suspect that your substrate at 62 mils is too thick, at least at the higher frequencies. As I recall this can cause strange propagation modes and non-physical effects such as you have observed, Try a substrate thickness on the order of 20 mils. The simulation tools should predict these modes and effects, so you can try it first in CST. You may see slightly higher insertion loss with a thinner substrate.
David

I did a Google search on Millitech GDM-12-WB-17M and found a datasheet for the GDM Gunn oscillator. series. The 17M refers to an output power of 17 milliwatts.
David