When users sets a 403 error page (Directory Listing Denied or Forbidden Access), Helm misses one of the 403 messages which happens to be the one that shows when no default documents are found. Unlike the 404 which is a single error code, the 403 has a bunch of variants, about 10 or so each for a different purpose.
Helm sets all the 403.X variants, except just one, which is the one for directory listing denied.
This is because Helm can not gather "default" error codes as they aren't in the metabase. You need to set a file value at the master site level and update all of the sites. This will set the error code into the metabase and will then get picked up by Helm.