This fixed all my problems!
Thank you @ncw !
It is amazing you guys thought about this issue and leave a flag/option to control it.
You rock!
You can see that now reaches 99%, and continues to the other remote.
For testing, I am using:
rclone sync /home/xxx/rclone_union/source/ hotmail_union:rclone_union --progress --log-file=/home/xxx/rclone_union/rclone_union_sync.log --delete-before --union-cache-time=0
I mean, for testing, I am using --union-cache-time=0.
For your information, I am uploading 50MiB files and I have a 1Gb/s connection. So, easily can exceed what is reported in the cache for free space.