I've been encountering this problem.
In my case, it strangely occured a few days before a concert, and during the concert !! What an ideal time for such a trouble, and the same repeated for a few concerts...
Aodyo's support was not helpful for me. I was so desperate I bought another Sylphyo and... had the same problem right away !
After careful investigation, I found what caused the problem and how to work around it.
It turned out that it was not moisture as such, the problem, but salt one. The one that is contained in sweat at the tip of the fingers. The kind of sweat one can have when being anxious before and at a concert !
I made some experiments, with normal water and salt water in contact with the body of the Sylphyo and the results is crystal clear : with normal water, no problem. With salt water, it is as if my finger was stuck on the key.
Also, I did the same experiments with normal/salt water inside plastic gloves : having the tip of the glove, filled with salt water (even with no finger inside) in contact with the body of the sylphyo would make the device consider that the corresponding key is pressed.
I'm surprised Aodyo's technical team don't seem to be aware of this electrochemical mechanism.
So, I know, now, how to work around it (using plastic gloves) but to be honest, I don't want to use Sylphyos anymore because of this. Lack of reliability which is a no-go in concert !