Failed Content Index State - Known Issue KB4045655

After installing KB4045655, the security update for Microsoft Exchange 2013 and 2016 you may encounter failed content index states.

View the status of all database content index:

Get-MailboxDatabaseCopyStatus *

How to recover from this can be done multiple ways, but the question I have is why did this happen in the first place?

Searching through the services list I came across the "Microsoft Exchange Search Host Controller" and it was in a Disabled startup type.
Strange because it needs to be started automatically as it interacts with the "Microsoft Exchange Search" service.

After setting the service to start as automatic began searching why it would set as disabled.

In the link I posted at the top of this post lays the answer.
And I quote:

Known issues

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We are aware of some reports that Exchange services may remain in a disabled state after installing this security update.  If this occurs, the update has installed correctly, but the service control script encountered a problem returning Exchange services to a normal state.  To resolve this issue, use Services Manager to restore the startup type to Automatic, and then start the affected Exchange services manually.

So there you have it, and it goes to show that reading the Microsoft documentation and knowledge base articles is a must.