The issue is the call to /ws/com.apple.CloudDocs/update/documents includes file_flags which defaults to is_writable: false.
The simplest possible reproduction is via rclone touch --dump=requests --log-level INFO iclouddrive:/Documents/unwritable-file.txt. Using the MacBook Finder application "Get Info" command will confirm the file is read-only.
I have submitted iclouddrive: fix so created files are writable by benalexau · Pull Request #8495 · rclone/rclone · GitHub in order to fix this.