when I try to convert an activity (phone call, appointment.... ) that contain a marketing activity as regardingobject to an opportunity, the field associated companies is empty
When you create an opportunity, the Related Campaign is not auto populated but the user can manually set the campaign. This has been like this in older versions or even the latest versions. Please see below
I do not think you can populate via mapping as its an N:1 relationship between phonecall and opportunity. If you would like to auto populate the value, you could think about creating a workflow / plugin .
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