Currently out of box, when lead is qualified and a contact/account is created, system manages "Originating Lead" column on contacts & account and prefill this with lead that just got qualified. This field is used for rolling up activities on lead to show up in Contact's timeline. But this works only for new lead getting qualified that results in new contact creation.

If we qualify another lead to an existing contact, this lead is not captured in Originating Lead lookup column. To my knowledge this is a root cause for allowing system to show lead activities on contact timeline and since originating lead only looks for first time lead, only first lead's activities gets rolled up but not other leads who are still connected to same contact.

Possible Changes :

1. In my view, if we change behavior for Originating Lead column to keep appending more leads based on how many gets qualified to existing contact, we can use and maintain this mapping.
2. Timeline Control - There could be some behavior change in timeline to look for multiple leads that would get captured in Originating Lead field and show activities from them on Contact and account timelines.