So, I know this isn't the answer anyone is looking for, but I posted above with the same issue, then suddenly (after a restart, and an update), the deleting thing stopped happening!
The ole' IT trick (did you try turning it on and off) kind of worked for me. (not immediately, but within a few days... hard to explain, it really did just stop happening).
That said, if you find yourself accidentally deleting emails from long ago, and its hard to find them in your deleted items, there is a way to sort your deleted folder on 'time deleted'. That way, you can sort the deleted folder by most recently deleted (regardless of the original email time/date), and it makes it easier to find what you accidentally deleted.
(just google how to sort liek that, as I don't recall how I did it; but i beleive tis a setting in Outlook)