Hi,
How many projects with this porfolio exist? You may be reaching the query limit. From my understanding, the default is 100 and max is 2,000.
You can define this with the query filter $$LIMIT, for example:
$$HOST/attask/api/v15.0/project/search?portfolioID=67263a2000d72af45a6ce21e558c51f8&$$LIMIT=1000
You can also narrow the list using status=CUR, etc.
Here are some resources that you might find helpful:
We've recently added External Lookups to our forms; they are a game changer! Good luck!