I can get that same result IF I connect a USB cable that that is data + power (VCC wire).
If I connect a USB cable that is Data ONLY (no VCC wire) so the battery is the only thing powering the seeeduino via the JST port , I get the 0.28V reading.
I want to program the seeeduino to send lorawan payload if the battery voltage drops which means there will be no USB connected.