What email client are you using to do this? It may be worth noting that writing the sent emails to a folder isn’t a server side function, and that can be confusing when the industry standard is considered Gmail which has an entirely proprietary system, where we use open source standards. Instead, for standard SMTP/IMAP, the sent email is written to a sent folder by the IMAP client over the IMAP protocol.
If two apps are considering the sent folder to be a different folder (for which there is no standard, see I can’t see mail sent from iOS Mail app in Roundcube - Question - MXroute Q&A, ) then you might be looking for the sent email in one folder and it might be in another.