dissolved brass fitting
I would think electrolytic corrosion to be unlikely due to the intermittent nature of condensate flow and the fact that the brass fitting is isolated between nylon and PEX.

Your thoughts around acidic condensate are intriguing. Boiler feed water for industrial steam systems is treated with inhibitors to prevent yellow metal corrosion in the condensate collection systems, and industrial closed-loop cooling systems are treated for the same reason, so it is a known fact that yellow metals are susceptible. Pure water easily absorbs carbon dioxide just from being exposed to air, so the gas-rich environment of the combustion chamber may be accelerating the process.

I would replace the nipple with a plastic component, that should take care of the issue either way.