Then it's quite obvious this happened. With an electrically conducive medium like tubeless milk (electrolyte) there is a electrochemical corrosion going on. The Aluminum has a lower valence than the carbon so there is a constant ions transport from aluminum to carbon. Ths makes the aluminum corrode. It really dissolves. This can even lead to a complete nipple head failure and the ripping apart of nipple head and rest of nipple. This also happens with water, but it takes longer as water is faster evaporating like the persistent milk.
ALWAYS use brass nipples when buildung carbon rims you want to run tubeless and you want to ride your bike also in wet weather conditions. Salt from the winter times, even help the corrosion.
The avaiation learned it the hard way, as they were placing aluminum ribs on bare carbon. Nw they are using a insolation layer between the both like glassfiber.

That's for dang sure. I wanted those rims this time, but as I build them my own, I prefer not to have to fiddle that much with all those nipples. As brass nor aluminum are magnetic, this won't help you here

Holy moly, these are looking bad. You had black sapim aluminum spacers under the nipple heads? And these looking great? Are you sure they are aluminum? You could try a magnet on them to find out.

My frame arrived!

Hope you all like the color, I think its fun!

Here is my clearance with the cheap ebay 165x42.5 2018 fox dps bottomed out. When its already bottomed out and I continue to bounce on it the frame doesn't budge. I agree with NumberZero and Medico, 42.5 seems bomber.

This build is looking better and better. No problems so far. Quality is great