This shows, that you use raid1 DUP, but that some of your data was not converted
This may be the right time to do a scrub to see if all data is OK !
And you should post also a
sudo btrfs device usage /
and later on maybe a (this one may take a lot of time)
sudo btrfs filesystem du -s /
you may have a look at:
btrfs wiki @ kernel.org
Did you interrupt your balance ?
OR: May be your volume is to small to complete this ?
How big is it ? Are you able to expand your volume ?
What about posting some usefull information ?
Provide Information:
Using RAID1 is preferable over using DUP. Especially when you are able to use 2 partitions on different disks !