One of the downsides to using multiple devices or clients against a single IMAP account is that every device/client does things a little differently. Some use the IMAP deleted flag, some use a “Trash” folder, others use “Deleted Items”. Sent it almost as bad, most clients use “Sent”, but many use “Sent Items” (Outlook’s standard).
The following link describes changing the folder for iphone mail:
https://www.everything-mdaemon.com/mdaemon/changing-the-iphone-imap-sent-folder-name