RAID6 scrubbing mismatch repair?

It is possible in theory: the data+parity gives you three opinions on what the data should be; if two of them are consistent, you can assume the third is the incorrect one and re-write it based on the first two.

Linux RAID6 does not do this. Instead, any time there is a mismatch, the two parity values are assumed to be incorrect and recalculated from the data values. There have been proposals to change to a "majority vote" system, but it hasn't been implemented.

The mdadm package includes the raid6check utility that attempts to figure out which disk is bad in the event of a parity mismatch, but it has some rough edges, is not installed by default, and doesn't fix the errors it finds.