It’s likely due to how folders are mapped. For some reason one device may be writing to one sent folder while another is writing to a different one. It’s important to note at this stage that sent email is not written by the server, rather the email client software pushes it to a folder over the IMAP protocol.
Try going into Roundcube webmail, click the three vertical dots by your email address at the top-left, and click on Manage folders. Look at what folders you have available, use the switch next to them to enable them in Roundcube’s regular view, then click Mail on the left side to go back to your inbox. Check out what folders you have any what may be where. Use that to determine which device is writing to what folder.
Here’s a quick guide to mess around with Sent, Drafts, and Deleted mail folder mapping on iOS: