I was testing with rclone copy. What mount with --vfs-cache-mode writes or full does to upload a file is essentially identical.
The test didn't complete because of a deadlock I found in the b2 code 
Here is a fix for that.
v1.65.0-beta.7390.2a30c417a.fix-b2-upload-url on branch fix-b2-upload-url (uploaded in 15-30 mins)