MacOS Mail delete function has a repeatable bug regarding deleting more than 1 message.

Boot into Safe Mode by booting with the Shift key held down and reboot normally. Then test to see if the problem is gone or not.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

No one in our family can reproduce your anomaly and none have encountered it. Using three laptops (two MacBook Pros, One Macbook Air, two iMacs. The iMacs and one laptop are used with a mouse and keyboard, two laptops are used with trackpad and built in keyboard. All have Apple Mail running. Never seen your issue. In Particular, my wife must get 300+ emails per day (don't ask why) and she spends a lot of time deleting emails one at a time or many at a time. She would have noticed this sort of thing.

Alternatively, your keyboard might be "sticky" with that delete key, although the fact that it also happens with the mouse-click-delete seems to contradict that possibility.

Both the mouse and keyboard have preferences that can modify how responsive they are and how quickly they "repeat." Perhaps you can experiment with those settings. You said it only happens in Mail but keep in mind that for some users, that is the app that makes the most use of the delete button.

Posting here is fine if you're just asking other users to verify/validate your findings, but if you're trying to report a bug, https://www.apple.com/feedback/ is the place to go, not here.