Using the Web UI, you can successfully update a feature, with the userId of a user who does not belong to the Program team but belongs to the Portfolio team.
This is not possible with the REST API as it will throw the error message:
"Cannot update feature with id XXXX. Value for ownerId is invalid."

Steps to reproduce.

1. Create a user (e.g. user1) under Administration>ACCESS CONTROL>People
2. Save the user called
3. Go to the Teams tab and add the user (user1) to a given Portfolio team (e.g Portfolio1 Team1)
4. Go to the Feature Grid and configure the tier 1 navigation bar to filter on the portfolio (Portfolio1 Team1)
5. Create a new Feature and set the owner to be user1
6. Save the feature
   No error message is given out.
7. Use the REST API to PATCH the feature to change for example the title.

   curl -X PATCH "https://<site-host>/rest/align/api/2/Features/<id>" -H "accept: /" -H "Authorization: bearer ******" -H "Content-Type: application/json;odata.metadata=minimal;odata.streaming=true" -d "[{"op":"replace","path":"title","value":"new title"}]"
   

   Actual Result:
   PATH Call fails with the error message: "Cannot update feature with id <id>. Value for ownerId is invalid." is thrown
   Expected Result:
   PATCH call succeeds with the update like the WEB UI

This makes the REST API nearly impossible to use for automation
We never know whether such "error" will be thrown, nor can any action be taken in the REST call.