[5valve]

I was thinking of head oring, it has two, bigger and smaller, both should be 1.78mm thick. If it is thicker, there is a compression blowby into gearbox, mixing coolant and gear oil. It could also push coolant into crank area, if there is a crack or bad seal between the cases. There could be only small coolant leak but in a longer timespan, this damage could happen. It is chronic, not acute occurance.

if bike is not flooded and is pressure cleaned correctly, there is no way, water could enter and corrode the bearings and crankshaft in just one instance.
I've cleaned other bikes numerous times without being extremely cautious, and nothing happened in years time.

This corrosion, on the other hand, has some deeper meaning. Same is happening to beta 300RR and xtrainer from 2015 till today. They even say 5-10% are affected. Its like enduro virus.

Beta's original response was: a bad batch of bearings, then changed to factory wrong bearing installation and again switched to bad bearing theory.
Beta uses NTN stock, so they say NTN is faulty. GG uses SKF explorer. And we are having SKFs fail...

I went for SNR and NTN combo, C3 & C4, but I think bearings are not to blame. Bad bearings fail in just couple of hrs.
Somewhere somehow there is a small leak into crank area, which in year or two kills the innards.

My engine is now apart for the first time, but you are having a 3rd occurence, therefore water entering somehow is the only thing I can think of.

Unless you are purposefully flooding the bike through carb and exhaust, not running it after washing and leaving it for weeks at a time, full of water....there's sth going on.