Update:
I was supplying regulated power (5V) via VUSB and GND. This afternoon, I found a power supply mistake in my wiring (which explains why the board got a bit hot).
The wiring was fixed and now I am having a stable mesh network. I will continue to monitor.
Here are the remaining problems:
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Sometimes, when updating Argon/Boron firmware, Xenons also flash Magenta!
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When Argon/Boron is reset (via System.reset and without power cycle), Xenons in the mesh never restart. They keep flashing cyan rapidly followed by a couple of yellow or red flashes, then fast cyan for a while .....