My understanding - non-expert:
It means that 6 sectors are causing issues - but haven't been mapped out yet (and won't be until TN tries to actually use those sectors for data)
It may (probably) mean that the disk is starting to fail. OTOH many people have used such disks for years without issues. YMMV
IMHO a modern disk (20TB is a modern disk) should not be showing any errors like that.
What would I do:
1. Replace the disk and resilver
2. Run a long smart test and look at the results (it may be that a long smart test detected these issues)
3. Run badblocks destructively on the replaced disk and see what happens and based on that, your appetite for risk, your level of resiliancy, your warranty and your backup availability consider wether you wish to keep using the disk or use it in a less stressful environment, or throw it in the bin
