I suppose I forgot to mention one important fact in my original post. When I first did a sync to these drives everything worked as expected. One drive filled up, then it moved onto a second drive and filled it up as expected. However trying to write anything NEW to the union causes the disk full error. My assumption is that on the initial check it sees the first disk is full and therefore errors out. However I don't know how it works so it's speculation. I think it was discussed in the other forum I linked which is why I forgot to mention.
All of the drives report correctly. Two of them are full from the initial sync. The about can be seen below:
rclone about gdrive1:
Total: 15 GiB
Used: 9.101 GiB
Free: 5.899 GiB
Trashed: 0 B
Other: 0 B
rclone about gdrive2:
Total: 15 GiB
Used: 14.968 GiB
Free: 33.111 MiB
Trashed: 0 B
Other: 0 B
rclone about gdrive3:
Total: 15 GiB
Used: 2.525 GiB
Free: 12.475 GiB
Trashed: 358 B
Other: 0 B
rclone about gdrive4:
Total: 15 GiB
Used: 14.967 GiB
Free: 33.767 MiB
Trashed: 358 B
Other: 0 B
All files I'm syncing are 50MiB so it's likely failing when it hits one of the 33MiB Free drives.
In the other forum you made a post about why determining no free space is difficult but had an idea to implement on a minfreespace flag where if the disk has less than the min configured free space to move to the next disk. Here is that post.