version: 6.8.0
we create a _default_ mapping of index template with the index sorting settings.
when we put data to create a new index of this template, then it will throw "unknown index sort field:[${index sort settings field}]"
we find the sources: MapperService.internalMerge() will not merge default mappings into fieldTypes, then the IndexSortConfig.buildIndexSort() would not lookup the field in fieldTypes