Water boils out so if it has been over 212 degrees since the contamination event the water is gone. This may indicate the issue is not a current one. Assuming it was a coolant mix that caused the issue. Otherwise it may have been a contaminated fill funnel/pump. The 500 PPM coolant sensitivity is a drop in the bucket. Although I don't see how it would have happened this is at the level for sample contamination.
Anything in the OEM oil that would trigger this?
All I can think of is a blown coolant hose or other leak somehow allowing coolant in through the breather. Just making sure these isn't any other ways for coolant to get in, known problems requiring a better hose routing etc.
The other concern is the 10 weight the oil sheared down too. Run way too long for sure.