Charging iPhone Triggers Checking Mail

Fetching mail has nothing to do with Background Refresh. When you plug it in all active apps are awakened to let them know. Mail (and other apps) see this signal as a wakeup and they will do whatever they would have done if you had launched the app.

You can’t. That’s the way it works, and has always worked. You can let Apple know you don’t like it—>https://apple.com/feedback - but, as yours is the first complaint I have seen in 13 years it probably won’t get much attention.

Whatever you think should happen isn’t happening, probably by design. No one here can do anything about it. If a lot of people sent feedback something may happen, but most complaints are the opposite, that Mail does not fetch when they think it should.