I've just upgraded to 6.1 (or done a data migration) and now many of our LWAPP APs are showing configs as Mismatch.
On the Audit page we see that for Secondary controller the Desired Config is listed as (emptry string) --which is correct because there is no Secondary controller-- but the Device Config is displayed as (failed to fetch).
This is due to a bug that will be fixed in a later version. The workaround is to run the following command on the AMP command line interface:
# echo "update device_config set value = '' where name like '%controller' and value is null" | db
[NOTE: The quotes after value = are two single quotes, not one double quote.]