Ok, here's the deal. There were files (older map versions) in the load that extended the size of the upload to well above the 2G limit. So the upload just dies with no indication. BAD!
So what makes this still hard to understand is that before I began the 1st upload attempt, I deleted all but the necessary map files before that upload and checked the directory size at under 1.4G. But I had to delete those very same extra files later after so many uploads failed! I can only think I must have somehow restored that full sized directory at some point but don't know how.

At least assets like models and materials will get automatically recompiled into the game directory if you view them in the asset browser. I haven't seen that happening with maps, but guessing it's possible some condition can cause them to get recompiled or repacked as well.

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