CHT remains low during flying at 16000 to 18000 ft altitude

Rotax specifies a maximum Cylinder Head Temperature (CHT), but are you measuring the material temperature of the CHT or the coolant temperature of the CHT, as seen in the newer-style cylinder heads?

Typically, people don't encounter issues with excessively cool CHT; the concern usually revolves around CHT running too hot, particularly at higher altitudes. In a liquid-cooled engine, CHT (Cylinder Head Temperature) readings typically trend hotter at greater altitudes, rather than cooler. This arises from the thinner air at higher altitudes, which demands the engine to work harder to sustain power, consequently generating more heat.

However, some questions warrant consideration:

Are all your CHT readings consistently cooler?

What indications are your EGT providing?

At altitudes that exceed the turbo boost range (turbo is too small for that altitude) you will lose manifold pressure and the ambient heat of outside is generally low.  In a piston engine the only thing you can do is use air blocking to reduce or block the incoming air that is cooling your radiator and perhaps even your oil.  Given you have to take off and get to altitude you need some type of adjustable system that you can open when you return to ground.  

What happens when it is too low?  Simply speaking the combustion inside the cylinder head is very poor.  The head no longer maintains the heat and deposits will increase due to rich fuel conditions with the very low temps.  At any altitudes over 8000 ft you should be using AVGAS regardless as it has a much better RVP and the octane is better for those conditions.  As Rotax owner  has pointed out be sure you are reading head temperature and not just coolant temperature.  There is a difference.  i would expect EGT will be low given the system cannot compensate for that low of an ambient temperature and cannot lean adequately  at high altitudes as the injected engine.  The exception to this is the type 915 and 916,, they have a larger volume turbo and are more capable at higher elevations. They also can keep the mixture ratio in a more optimal range.  The 914 is tapped out at about 5000M, about 16,000 ft density.  Beyond that the turbo can no longer compress the air enough to maintain max continuous so you will drop power. 

Cheers