It's really a matter of electrical/mechanical characteristics and design constraints. We wanted to make the Prophet 12 Module as small as possible, which we did. This, like many design decisions comes at a price. The Prophet 12 main board runs pretty warm and there's no way around that. So, we commissioned a professional thermal analysis which gave us the best possible heat dissipation options for the enclosure through passive convection. To further aid heat dissipation we used a carefully positioned thermal pad.
Thermal pads are widely used in various electronics applications. A quick internet search will garner a myriad of results showing similar behavior of thermal pads over time, i.e. secretion of inert, non-conductive fluid. The "expected behavior" you are referring to is that thermal pads have been known to express this fluid, not that "we expected fluid to leak out of your synth". We're not trying to hide anything from anyone and we're also not excited about this.
Fortunately, it does not affect the operation of the Prophet 12 Module, and it is easy to wipe away if you see any fluid. It is unlikely the amount of fluid emitted from the thermal pad will amount to much more than a tiny spot on the outside of the enclosure, if at all. It's not going to drip on other gear, it just doesn't have that capacity. It's also not necessary to open the enclosure to clean the fluid. If you see it on the outside, simply wipe it away. There is a possibility you may never encounter it, too.
We *do* apologize for any inconvenience this may cause or have caused you. There's not really much more we can say, other than we used a thermal pad for heat dissipation instead of making the P12 Module larger and possibly non-rackable as an alternative, so we went with keeping things small. Hopefully this sheds some light on the situation.