What is the source of the moisture?.....Is it condensation in the gear case?.....
If so, are idle aircraft at greater risk?
If the air volume within the transmission housing contains moisture then the moisture can condense and form droplets on the internal transmission surfaces when things cool off after shut-down. If these moisture droplets work their way down into the oil sump then they do not present much of a problem. But if these droplets collect on critical surfaces of highly stressed ferrous components like gear flanks or bearing races (or even a bevel gear shaft) and remains there for even a short period of time, the subtle surface corrosion damage produced can easily be sufficient to initiate premature structural failure of the components.
For this very reason, it is now common design practice to force all air flowing into the transmission housing to pass through
a breather device that employs a desiccant element. After a few heating/cooling cycles, the air volume within the transmission housing has been thoroughly de-humidified. I don't know whether the EC225 transmission uses such a device. Based on the reports describing corrosion from internal moisture as being one potential source of the problems, then I would assume that no such desiccant breather system was used on the EC225 MRGB. Maybe someone with a detailed knowledge of the EC225 MRGB design can provide clarification.