What's the Time Machine bug? I use Time Machine.
I'm still pursuing it. I've completely reproduced it with fresh Time Machine volumes on Sequoia and Tahoe and I've reproduced it with 5 separate drives. I can only comment on the state of affairs for APFS. You will not notice the bug if you don't monitor your CPU, but the bug will cause significant wear if you are backing up to SSD's.
It's off-topic for this thread since it impacts Sequoia.
In a nutshell, every time you mount a volume that is configured for TM (whether or not the machine is configured to use it), Spotlight will index the oldest backup snapshot found on that volume. Every time it does this, it updates the spotlight index at the root of the TM volume.
So it's indexing the content found at:
/Volumes/.timemachine/<volumeId>/<oldest backup time>.backup
and updating the index at
/Volumes/TMDrive/.Spotlight-V100
That oldest backup never changes, so re-indexing it on every mount is mysterious. I've seen re-indexing take anywhere from 1 minute to many hours.
This morning's testing is confirming that the problem is completely reproducible on Sequoia. I created a brand new TM volume on an SSD and did a full backup of my Sequoia machine. I then ejected the drive and reinserted it. It's been reindexing for the last 2.5 hours.
Tahoe behaves the same way. One significant step forward is that the Tahoe logs show more errors relating to the state of the Time Machine backup and might provide a hint as to what's going wrong. At the root of the drive there will be a single backup in a folder named <...>.previous. I believe the snapshots on the drive are snapshots of that. The logs show that the state of that .previous folder is off. That could be what is triggering the problem.
You will find tons and tons of people complaining on the internet of Spotlight indexing Time Machine backups. But the complaints are sometimes vague and rarely resolved. People don't explore this issue thoroughly. Most people don't even notice that Spotlight is repeatedly indexing. The fact that the process repeats after every reboot means that suggestions to reboot to solve some problem could be counterproductive for TM users.
I've given up using Time Machine. I'm only continuing to explore the issue to learn. I have Carbon Copy doing hourly backups with snapshots, and it's working flawlessly.