Union remote of google team drives does not work with --drive-server-side-across-configs flag

Output of rclone version

rclone v1.58.1

• os/version: ubuntu 20.04 (64 bit)
• os/kernel: 5.13.0-51-generic (x86_64)
• os/type: linux
• os/arch: amd64
• go/version: go1.17.9
• go/linking: static
• go/tags: none

Describe the issue

I and many others have noticed that if you wanted to sync a union of 2 team drives to either another google drive union mount or an individual google team drive while using the flag --drive-server-side-across-configs=true, rclone ignores the flag and downloads the files to the endpoint that calls the command and then uploads it to its destination.

Here is an example of my rclone config and the command:

rclone copy 'teamdrive-combo:TestFolder/large20GB.txt ' TeamDriveC:clones --fast-list --transfers=16 --checkers=32 --tpslimit-burst=16 -vP --ignore-errors --stats=30s --max-backlog=2000000 --ignore-case --size-only --no-update-modtime --drive-chunk-size=256M --drive-use-trash=false --use-mmap --drive-server-side-across-configs=true --drive-stop-on-upload-limit --drive-skip-gdocs --timeout=10s --tpslimit=4

rclone config:

[teamdrive-combo]
type = union
upstreams = TeamDriveA: TeamDriveB:

[TeamDriveA]
type = drive
client_id = xxxxxxxxxxxxxxxxxxxxxxxx.apps.googleusercontent.com
client_secret = xxxxxxxxxxxxxxxxxxxxxxxx
scope = drive
token = {"access_token":"xxxxxxxxxxxxxxxxxxxxxxxx","token_type":"Bearer","r>team_drive = xxxxxxxxxxxxxxxxxxxxxxxx
root_folder_id =

[TeamDriveB]
type = drive
client_id = xxxxxxxxxxxxxxxxxxxxxxxx.apps.googleusercontent.com
client_secret = xxxxxxxxxxxxxxxxxxxxxxxx
scope = drive
token = {"access_token":"xxxxxxxxxxxxxxxxxxxxxxxx","token_type":"Bearer","r>team_drive = xxxxxxxxxxxxxxxxxxxxxxxx
root_folder_id =

[TeamDriveC]
type = drive
client_id = xxxxxxxxxxxxxxxxxxxxxxxx.apps.googleusercontent.com
client_secret = xxxxxxxxxxxxxxxxxxxxxxxx
scope = drive
token = {"access_token":"xxxxxxxxxxxxxxxxxxxxxxxx","token_type":"Bearer","r>team_drive = xxxxxxxxxxxxxxxxxxxxxxxx
root_folder_id =

Start by reproducing the command with the union remote, Google team drives, and --drive-server-side-across-configs=true. Trace the union remote and Google Drive transfer handling to find where the flag is ignored; done means the transfer remains server-side rather than downloading and re-uploading through the client.