mmm you them may have enabled Auto Archive.
From reading other threads on this forum, this issue might be related to “auto archiving” if you have that enabled in eM Client settings ?. You can check if its been enabled via “Menu / Settings / Mail / Automatic Archiving”.
As Gary explains at the end of the post extract below, “You can’t rename the Archive folder”, but you can create a new folder within the archive and manually move selected files to that folder".
See @Gary post below from the following thread.
https://forum.emclient.com/t/how-does-auto-archive-work/77348
How does auto archive work?
Automatic Archiving moves messages based on age from IMAP folders to a local folder Archive. This is done on a schedule.
The destination is a duplicate of the folder structure on the IMAP server. So if the message to be archived is in the Inbox on the server, it will be in the Inbox in the archive. If it is in the Sent folder on the server, it will be moved to the Sent folder in the archive.
This does not create separate folders for each year, but simply duplicates the structure from the server.
You can’t rename the Archive folder, but you can create a new folder within the archive and manually move selected files to that folder. The Automatic Archiving will not add messages to that folder though