Hi Sarah Stevenson,
Did you ever get this figured out? We're having the same problem as well.
No its never been figured out ... sorry 😞
Doesn't seem like Marketo has given much thought to their Salesforce app in sometime either...
This may be a Salesforce issue. I know that we had it set up to the default with Salesforce where activity data could only be reported on if the activity took place within the last 365 [rolling] days. I know this is specifically for reports, but you may want to contact Salesforce support and ask them to extend the data availability to at least 3 years. Not sure if this would work for this issue, though.