Problem with MagSafe Duo and iPhone 12 Pro

Situation: Recently upgraded iPhone 12 Pro to iOS 15.1. Have MagSafe Duo Charger, but only use it while traveling. A day ago I used it to recharge the iPhone during the night. The next morning, assuming, as had always been the case in the past when I used this setup, that the iPhone was fully charged, I put it in my pocket, only to discover a few hours later that it only had 23% charge. When I arrived back at my room, I connected it directly to the Apple 20W USB-C power adapter until the battery reached a reasonable level (around 80%). Curious, I then connected the MagSafe Duo to the adapter and put the iPhone on it. Despite a reassuring reception from the Charger, I noticed that the charge dropped steadily. When the iPhone had lost 10% of the charge, I assumed that was the explanation to the 23% in the morning. It seems the MagSafe Duo Charger is sucking power from the iPhone's battery. I am at a loss to explain this, but perhaps someone can.