Large BTRFS array suddenly says 53TiB Free, usage inconsistent

Joshua
2021-11-14T05:51:10Z

I have a large multi-device BTRFS array. (13 devices / 96TiB total usable space)

As of yesterday, it had a little over 5 TiB reported as estimated free by 'btrfs fi usage'

At exactly 7am this morning, my reporting tool reports that the "Free (estimated)" line of 'btrfs
fi usage' jumped to 53TiB.

Now I do use snapshots, managed by btrbk. I currently have 80 snapshots, and it is possible old
snapshots were deleted at midnight, freeing up data. Perhaps the deletions didn't finish committing until 7am?

However, the current state of the array is concerning to me:

#> btrfs fi usage /mnt
Overall:
Device size: 96.42TiB
Device allocated: 43.16TiB
Device unallocated: 53.26TiB
Device missing: 0.00B
Used: 43.15TiB
Free (estimated): 53.27TiB (min: 53.27TiB)
Free (statfs, df): 4.71TiB
Data ratio: 1.00
Metadata ratio: 1.00
Global reserve: 512.00MiB (used: 0.00B)
Multiple profiles: no

Data,RAID1: Size:43.10TiB, Used:43.09TiB (99.98%)
{snip}
Metadata,RAID1C3: Size:66.00GiB, Used:62.51GiB (94.71%)
{snip}
System,RAID1C3: Size:32.00MiB, Used:7.12MiB (22.27%)
{snip}
Unallocated:
{snip}

As you can see, it's showing all my data is Raid1 as it should be, and all my metadata is raid1c3
as it should be.
BUT it's showing data ratio: 1 and metadata ratio: 1
Also, the allocated space is showing 43 TiB, which I know to be around the actual amount of used
data by files. Since Raid1 is in use, Allocated data should be around 86....

Any ideas as to what happened, why it's showing this erroneous data, or if I should be worried
about my data in any way?
As of right now, everything appears intact....

--Joshua Villwock