The right earphone of the Airpods has a low battery even in the charging case

My Airpods right earphone dropped in battery after a night of inactivity.

After I opened my Airpods box I had to wait until my Airpod recharged.

I tried the options:

• Charge my airpods overnight
• Hold the reset button for the airpods for 10 seconds
• Turn off bluetooth of iPhone

But the above phenomenon still occurs. And so every morning when I wake up I have to turn on my airpods so that the right earphones can be recharged. I discovered my iPhone can connect to the airpods even when the airpods is not open. Is this a firmware error?