@EricRegnier I've just checked and in (an old) D365:-
Contact record - activities are shown for the account when it's set in the contacts account field
Account record - activities for the primary contact are shown on the account form.
But as you say and I've just double checked the "cross over" information isn't shown in a none D365 power apps.
Now my go to check for anything that is different between worlds which my be a settings flag is Sean McNellis's OrgDbOrgSettings powertool and looking at the history there is an April 2019 flag called
- EnableActivitiesTimeLinePerfImprovement so I wonder if that is the difference.
Edit - just tested on a more recent environment and it's consistent - D365 carries activity details between primary contact and account, PowerApps doesn't