Hmmm... Where to start?
Debugging a humidor is not really the easiest process. Huge assumptions need to be made. Size, seal, circulation, engineering, controls... the ambient... etc. There are many, many factors.
Assuming the unit is sealed well and is not unduly influenced from outside factors (a reasonable delta to the world outside) then the humidor only need overcome realistic variables. Anyone who knows of what I do, knows I toss my projects into the realm of the elements to prove they work. In a perfect world, no humidor is necessary at all. PigFish's First Law.
Next... I have a pretty good idea what is going on here, but I cannot write a book about it here. Time is a problem and the theft of some of my original ideas another. While I have typically shared openly ideas and solutions, I have come to find that some 'folks' are attempting to knock off some of my designs. While that is the price of progress, frankly I am not going to do the research and give others a free pass on some of the best solutions. Many ideas I have published on my YouTube site.
One problem you have is the use of isolated components and not using a consummate unified controls systems. Advanced humidor controls equipment and appliances need to know what each component in the system is doing otherwise you fall back to the earliest generation of humidor control systems which did not work so well. You are currently working on the most basic level of control (and engineering). This is one reason why you are facing 'early systems' failures. I work at an extremely high level here where many of you will (likely) not believe that the end result justifies the means, the cost and complexity. This is why I am a humidor 'nerd' and most cigar smokers are not.
Separating 'functions' then, is a primary aspect of humidor engineering and control. When you buy a split AC system, designed for human comfort, whereas the human will simply reduce the set point to the level of comfort, you get systems designed to work for the human comfort mindset and not the humidor controls mindset.
A humidor does not function in the realm of a 'human comfort' mindset. It must function on the level of a scientific instrument and control mindset. They are totally different. An air conditioner controls heat via a primary process and rH via a secondary "coincidental" process, not suitable for cigars. This will likely add up to failure, at least in a smaller environment where the AC unit will likely be far more powerful than is necessary to control the limited space necessary to condition cigars. The air conditioner maker was likely not involved in what was to be done with the system you purchased (humidor heat control). Someone knowledgeable about humidor systems is a different story. Frankly, if inquiries were made, someone should have told you that this ideas was at least potentially problematic. In many cases, the layman would not know what questions to ask and likely the salesman would not know the answers as the systems that are mostly sold are sold around the "human comfort" mindset.
So, you have both an engineering and controls problem. You have systems as they stand that are likely incompatible. Empirically, you knew this with out all my pixels... I am sure.
For your enlightenment I have included the video below. This video, will actually visually demonstrate all the real world (controls) solutions to your problems, not specifically fit to your system. Integrating your system may or may not be so easy. Likely easy for me, I don't mind cutting out whole controls and wiring my own. However, buying fixed, proprietary, integrated systems for humidor use is almost always a hurdle, even for me.
Watch the video...!
If you would like a more specific answers you need to contact me directly.
Fair warning... I have tons of unanswered emails. I am simply too busy at my 'real' job to take on too much work. I have to process a lot of technical email for a living. I am almost always behind.