Can't delete Btrfs filesystems set up with RAID (device busy)

[HarmfulBreeze]

Explain what happens

I discovered this bug during the Anaconda Web UI Test Day, so the steps make use of Anaconda Web UI too.
According to the Anaconda dev team, this is a cockpit-storage bug.

1. Install Fedora following these steps. You'll end up with a RAID1 Btrfs setup: https://fedoraproject.org/wiki/QA:Testcase_partitioning_webui_guided_multi_empty_all
2. Try to install Fedora one more time by following this other procedure, using either disk: https://fedoraproject.org/wiki/QA:Testcase_partitioning_webui_custom_standard_partition_xfs

You can't go past step 5 of the test case, as you can't remove the replicated filesystem (this device is currently in use).

Version of Cockpit

343

Where is the problem in Cockpit?

Storage

Server operating system

Fedora

Server operating system version

Fedora-COSMIC-Live-Rawhide-20250805.n.0.x86_64

What browsers are you using?

No response

System log

journal.txt

[HarmfulBreeze]

I was able to work around the issue by unlinking the two filesystems. Afterwards, I could remove the Btrfs filesystem from both disks using the storage editor.

# mount /dev/sda3 /mnt
# btrfs balance start -mconvert=dup -dconvert=single /mnt
# btrfs device remove /dev/sda3 /mnt
# umount /mnt