lastHealthyBlock is never populated

Do you want to request a feature or report a bug?
Not sure. Seems like a bug to me

What is the current behavior?
For all the subgraphs I have deployed so far, I have never seen lastHealthyBlock be anything other than null. Not sure if that's supposed to happen and if yes then why is the key in the first place?

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem.

1. Deploy the gravatar subgraph from the docs
2. Run the following query

req := graphql.NewRequest(`
		query {
			indexingStatuses {
				subgraph
				synced
				health
				entityCount
				node
				fatalError
				nonFatalErrors
				chains {
					network
					chainHeadBlock { number }
					earliestBlock { number }
					latestBlock { number }
					lastHealthyBlock { number }
				}
			}
		}
	`)

3. Notice that lastHealthyBlock is null

What is the expected behavior?
That lastHealthyBlock should be populated

Reproduce the report by deploying the gravatar subgraph and running the provided indexingStatuses query. Trace how the chain status fields, especially lastHealthyBlock, are produced and compare that behavior with the expected populated value. Confirm the result against the same query after the change.