OK, I can't explain that outcome. Too bad.
Either way, I suspect self-discharge of your batteries is the main culprit. Try your light meter(s) with a regular alkaline battery and see how it goes. If the problem goes away, you know what the cause was.
Either way, I suspect self-discharge of your batteries is the main culprit. Try your light meter(s) with a regular alkaline battery and see how it goes. If the problem goes away, you know what the cause was.