2 realtime notification emails from the same document but on different XWiki instances are grouped under the same thread

How to reproduce:

1. Start instance 1 and start watching Sandbox.WebHome
2. Start instance 2 (on the same or on a different machine) and start watching Main.WebHome with a user having the same email as in the previous step
3. With a different user, make a change on Sandbox.WebHome on each instance
4. You will receive 2 notification emails for the same document that will be grouped under the same thread

Expected:
Each document should have its own thread for these and future modifications because they belong to 2 different instances so they are 2 different documents, even if they have the same reference (wikiId+spaceId+nameId)