The portal we see with snapstart is not really the portal the developers configured (in terms of dynamic container config) When contents is loaded through SnapStart it loads them in frameless portal.
Snap-start does not define portal type as it is a authentication mechanism. Snapstart renders the UI in frameless container.
So the original Enhancement was to support a URL that uses snap start to open a specific work object in the context of the portal was not possible to achieve. The user would potentially have to authenticate before the work item is loaded (or a SSO setup would do that authentication based on the deployment).
Under an open enhancement request EPIC-37111 (Launch links in the context of the user's portal context) the product is waiting for functionality that will allow a user whenever they click a link to open up a work item from an email , to open up as a new document in the user's portal context.
For now the functionality will launch in the user's default portal (Case Worker or Case Manager).