I had a go at setting visibility - can you try this?
v1.56.0-beta.5372.db1521f29.fix-dropbox-link on branch fix-dropbox-link (uploaded in 15-30 mins)
Note you'll need to set --expire in order to set visibility to work around a dropbox SDK bug.
You can set --expire 10y or something like that though.
If it doesn't work can you do the rclone link command with -vv --dump bodies and post the result?