As far as I know restic validates that cache is still valid (not removed). But to be 100% sure, just remove it locally.
rebuild-index and then backup --force should fixe such issues (if files are still present and not modified).
Are you sure that file is not changed? Some times ago there was topic here that MS Excel may silently modify file (even if you don’t change it and don’t trigger ‘save’ action). And even more, to hide this, excel restores ‘modification time’ of file so that this change is not visible: