iPhone 12 mini battery is draining when charging with MagSafe (with 20watts adapter) wireless charger and with phone being idle

I just bought the new iPhone 12 mini, switched from my iPhone 11 Pro. Very excited. I updated to iOS 14.3 and later to iOS 14.4 public beta (both versions encountered the same issue described below).

I also bought the official Apple MagSafe Wireless Charger and the 20Watts Apple Adapter.

However, when I try to charge the phone during the night with MagSafe for the first time, I found the battery icon is indeed green, meaning it was charging. But to my big surprise, the battery is draining. As shown in the screenshot below. The battery is literally draining while charging, and the phone was screen-off doing nothing. I thought it was one incident, but again, it happened the next day, as you can see another charging with a draining battery around 18h in the screenshot.

However, I can normally charge with the USB-C to lightning cable, with the same 20Watts adapter.

I did a remote diagnostic with Apple Service and there's no problem with the hardware of the iPhone 12 mini.

This means that either

• there is a software issue triggered by MagSafe Charging, causing battery draining while charging (perhaps reverse charging???)
• the MagSafe wireless charger is fake (that I am not sure, but it appears to be real). I attach the photos in case anyone can offer a clue.

I am really deeply puzzled. I was so excited about MagSafe wireless charging but not really disappointed.

Thank you for any clue!!