`btrfs scrub status` behaving strangely

[rrueger]

Running Ubuntu 20.04.6 LTS with kernel 5.4.0-156-generic

I began a scrub, but it appears to be "stuck" with the following symptoms

1. I began a scrub with btrfs scrub start -Bd /btrfs
2. The scrubbing process appears to be working
3. iostat is reporting lots of disk activity and scrub status is as expected
4. At 6.56/8.79TB it seems to hang
5. iostat is reporting zero activity on the disks, no reads, no writes, no nothing
6. However, scrub is saying that it is reading at 100MB/s

Here is some sample scrub status output

Scrub started:    Sun Aug 13 23:19:12 2023
Status:           running
Duration:         43:43:23
Time left:        14:53:42
ETA:              Wed Aug 16 09:56:17 2023
Total to scrub:   8.79TiB
Bytes scrubbed:   6.56TiB
Rate:             43.70MiB/s
Error summary:    no errors found

It has been outputting information like this for around 12-14 hours.
The rate keeps going down over time (it is not statically printing the same every time)

Cancelling the scrub just hangs

This happened before and presumed this was a different error

btrfs check returned no errors on the disks

smartctl -A returns nothing super suspicious

The disks just seem to completely lock up, even though no IO is happening