I'm using HubSpot's Gmail integration to send emails from within hubspot.com. These emails typically all have the same subject line. Gmail is grouping all of these HubSpot-originated emails into the same conversation, even if they are with different people.
After some investigation, I've found that Gmail's incorrect grouping of these emails into the same conversation is because HubSpot is sending these emails with incorrect headers.
In the emails that HubSpot sends for me, the headers in the email all have the exact same ID in the "References" field of the header. This tells Gmail that all those emails belong to the same conversation, when they really don't.
I only just started using HubSpot so I can't say how long this bug has been happening. It is seriously negatively impacting my workflows. (I was able to somewhat work around it by disabling "conversation view" in Gmail, but that's not ideal operationally either; being able to use conversation view would be great.)
We are also experiencing a similar thing. I sent a ticket to Hubspot's support inbox and mentioned this thread specifically.
Essentially emails were sent in my case from Hubspot, and in hubspot they show up properly. In gmail, gmail will group the same title emails together in a thread but have contacts from completely unrelated companies.