hi, here also using Spark and same problem.
I just talked to Hubspot support and we found out a temporary though not ideal solution that works
Just add BCC your Hubspot account ID (HubspotID@bcc.hubspot.com) when you send an email from Spark and this subject thread will get permission to get registered in Hubspot. Take into account that each different subject needs permission, so each time you send an email to the same person with a different subject you have to add BCC.
Eager to have a proper integration anyway 