Please understand that RF oscillators are never perfect. They tend to drift a little with temperature and with humidity. On a 500kHz oscillator, a temperature drift of +500Hz represents an "error" of 0.1% which would be negligible in most technical applications, but which is already crucial in a theremin where it would lead to exactly that field shrinking which you describe.
That's why in most theremins, both pitch oscillators, the fixed one and the variable one are built with identical components and together in a cabinet. Thus, temperature changes would affect both, and even if both went up by 1% or 5kHz, you wouldn't notice it, because the resulting beat frequency (the difference between both) would remain nearly constant.
It must be said that most of the temperature drift in theremins happens during the warm-up period. It takes a few minutes until the current flow through the components has heated them slightly up and they arrive in a thermally balanced and stable state. That's only the moment when the player should tune or calibrate their instrument, because afterwards, the remaining temperature drift is much less.
With the OTv4, things are a little more difficult. First, both pitch oscillators are not identical and won't have common temperature drift since the fixed one is a rock stable Xtal oscillator with almost no noticeable drift while the variable oscillator is (to make it variable through the antenna capacitance) a classic analog circuit, even though it's build with unbuffered CMOS gates, which loves drifting with temperature. That's why temperature and air flow considerations are more important here than with some other theremins.
As you already found out at home, a constant ambient temperature helps greatly for thermal stability. But outside in the wild "with the nude butt in the wind", each slight breeze will de-tune the pitch oscillator and since it is in constant exchange with the ambient air, it will never reach a thermally balanced and stable state. What to do?
First, don't expose the OTv4 to the free air. Build a cabinet around it which will allow it to have its own micro-climate and which will make that after a few minutes of warming up, a thermal balance between the heat from internal power dissipation and the cold from outside will establish itself. Then only, launch the calibration process and play happily afterwards! 
