This is expected behaviour by design.
Teams Chat Archive is intended as a point-in-time archive operation. The "Rearchive and overwrite any previously archived chat file in the target tenant" option is designed to recreate and replace a previously archived file, such as when the original archive file has been deleted or needs to be regenerated.
The rearchive process recreates the archive based on the content that was previously archived and does not append or include messages that were added after the initial archive was created.
There is currently no supported method to update an existing Teams Chat Archive file with newer messages through subsequent archive migration runs.
If an archive containing newer chat content is required, a new Teams Chat Archive migration can be performed in a separate project to generate a new archive based on the current chat content. Please note this may result in a duplicate archive file.
Teams Chat Archive should be treated as a one-time archive operation.
