I have Apple Mail set up on my mobile devices to only retrieve mail manually.
Settings–>Mail–>Accounts–Fetch New Data–>Off
Account–>Manual
Fetch–>Manually
This behaves correctly on my iPhone SE (2016). It does not behave correctly on my iPad Mini 5. Both devices are running IOS 14.8.1, have SIM cards, and are connected to WiFi.
As a test, I shut down the iPad – restarted the device – and did not open the Mail app at all.
For the first two days all was fine – no new emails. On the third day there were new emails as indicated by the badge on the icon. I have repeated the above test a couple of times and after a few days it retrieves mail. As part of this test I do not open the app after a restart so why is Mail retrieving email?
Try quitting the Mail app: on older iOS devices double click the home button to display running apps, locate the Mail app and swipe it upwards to quit. This sometimes works better than a restart.
I suspect, however, that you have come across a bug with iPadOS that is unlikely to be fixed by Apple. You can always try submitting a bug report to Apple but they don’t seem to encourage feedback for Mail on this page:
FWIW this has been the case with my iPhone for years (always just once or twice a day, every day), through multiple iOS updates. I eventually gave up trying to resolve it because it doesn’t present a problem other than being annoying. I didn’t report it as a bug.
What happens if you turn off all notifications for the mail app? If the app is receiving notifications, I wonder if that is triggering it to update messages.
One other thing that you can try is “Automatic” rather than “Manual”. According to what Apple says at the bottom of the device, that will retrieve mail only when on both power and WiFi. So, you’ll still have mail fetched when you plug in and are on power and WiFi, but otherwise when it is not plugged in, it should only update when you actually open the app, even when on WiFi. (Unless there is some reason why you don’t want mail fetching when it is on power? Then I’d try turning off WiFi when you plug in.)
Same problem here. Has been that way on various iPhones for as long as I can remember. I really don’t get how Apple can deliver stuff that is broken on such a basic level and then refuse to fix it over many generations of iPhones and iOS. Seriously, how hard is it to just not download mail until I tell it to download? Really doesn’t sound like rocket science. But obviously it takes much more engineering prowess than releasing 1,500 new emojis every other week.