Exchange AD topology does not discover all DC from all AD sites

Symptoms:

Exchange 2013 AD topology discovery runs every 15 minutes and discovers all the In-site Domain Controllers/Global Catalogs, but Out-of-site Domain Controllers/Global Catalogs are listed only from one another AD site.

First discovery finds all In-Site DC/GC from New York site (NY-DC & NY-DC2) and lists only DC/GC from London site (LN-DC1 & LN-DC2)

Log Name:      Application
Source:        MSExchange ADAccess
Event ID:      2080
Task Category: Topology
Level:         Information
Keywords:      Classic
Computer:      NY-Ex1.contoso.com
Description:
Process Microsoft.Exchange.Directory.TopologyService.exe (PID=1234). Exchange Active Directory Provider has discovered the following servers with the following characteristics:
(Server name | Roles | Enabled | Reachability | Synchronized | GC capable | PDC | SACL right | Critical Data | Netlogon | OS Version)

In-site:
NY-DC1.contoso.com CDG 1 7 7 1 0 1 1 7 1
NY-DC2.contoso.com CDG 1 7 7 1 0 1 1 7 1

Out-of-site:
LN-DC1.contoso.com CDG 1 7 7 1 0 1 1 7 1
LN-DC2.contoso.com CDG 1 7 7 1 0 1 1 7 1

Next discovery finds all In-Site DC/GC from New York site (NY-DC & NY-DC2) and lists only DC/GC from Tokyo site (TK-DC1 & TK-DC2)

Log Name:      Application
Source:        MSExchange ADAccess
Event ID:      2080
Task Category: Topology
Level:         Information
Keywords:      Classic
Computer:      NY-Ex1.contoso.com
Description:
Process Microsoft.Exchange.Directory.TopologyService.exe (PID=1234). Exchange Active Directory Provider has discovered the following servers with the following characteristics:
(Server name | Roles | Enabled | Reachability | Synchronized | GC capable | PDC | SACL right | Critical Data | Netlogon | OS Version)

In-site:
NY-DC1.contoso.com CDG 1 7 7 1 0 1 1 7 1
NY-DC2.contoso.com CDG 1 7 7 1 0 1 1 7 1

Out-of-site:
TK-DC1.contoso.com CDG 1 7 7 1 0 1 1 7 1
TK-DC2.contoso.com CDG 1 7 7 1 0 1 1 7 1
Resolution:
This is by design in Exchange 2013.
Previous versions of Exchange server discovered all Out-of-site Domain Controllers from every other AD site that had direct AD link defined. This was found to be resource intensive and time consuming therefore starting with Exchange 2013 randomly selects an AD site to use the Domain Controllers in case In-site Domain Controllers became unavailable.