Hi,
I tried using the callback but it didn’t seem to work, in the setup function I use LowPower.wokeUpFromDeepSleep() to recover the previous state. This issue only happens when using Serial.begin and connecting and then disconnecting USB. Without Serial.begin it works fine. After a power cycle with Serial.begin but without USB connected at all it also works fine. Plugging in USB makes the red LED flash dimly and then the wake failure becomes an issue but only after unplugging USB. I suspect the USB stack.
This is a project I’m helping a friend with, I’ve been working as an embedded C developer for 8 years doing board bringups and advising schematic design. Documentation can’t always be trusted…