external battery

I have had a problem with the use of external batteries that happens EVERY. SINGLE. TIME. I don't understand why it never seems to happen to anyone else. If I ever dare use an external battery, my internal battery becomes useless. It will go from full-charge to shutdown in a few hours.

Here are the factors that don't seem to matter:

• The brand of external battery. Cheap, expensive, well-reviewed/regarded, fly-by-night. It does not matter.
• The brand of cable used. Cheap, expensive, generic, Apple-MFi, or Apple-branded. It does not matter.
• Version of phone. It happened on my 3, 4S, 6S. It does not matter.
• Age of the battery or phone. New, aged. It does not matter
• Version of iOS. It has probably happened on each version including dot updates. It does not matter.
• Length of time on the charger. Full charge or ten minutes. It does not matter.
• Length of time between uses of external battery. Everyday or once a year. It does not matter.

I have avoided the use of external batteries for months or years at a time because of this issue and then I will try it again because it's a different phone and no one else ever complains of this. Burned again.

Two days ago, I tested an Apple-branded (purchased at the Apple store) Lightning-to-USB-C cable between my iPhone 6S and my iPad Pro 11-inch 2018 to top off the battery. The battery started tripping out, so I drained it until it shut down. (It didn't take long.) Last night I had it connected all night into one of my reliable wall-plugged adapters that I have used for years. I unplugged the phone at 7am and by 11am it wanted to go into low-power mode at 22%. After being plugged into wall-plugged power, for 10 minutes, it says 55% a few minutes later, 61%.

I don't use backgrounding. My battery behaves OK outside of this problem without me changing my habits.

There are only two things that have ever fixed the problem. 1.) Take it to the Apple store where they poke at it for 5 minutes. 2.) Wait an indeterminate, random amount of time (days or weeks).

Am I really the only one in the world to experience this?

Is someone gaslighting me?

Is there a way to get my battery back on track without visiting the Apple store or waiting?

Here you can see the battery dropping 25% in 12 hours without being used. The you can see it rise as it's plugged in overnight then drop to 18% in 4 hours after taking some photos and looking at Twitter for a few minutes: