The D365 Portal needs to be configured to automate the clearing of the cache within the Portal for data that is modified in D365. As a Portal Administrator you can access a hidden portal page, once you have logged in, to manually clear the cache. After logging in, navigate to <portal url>/_services/about. On the page, in addition to some details about your portal is a Clear Cache button which forces the portal cache to be refreshed.

https://docs.microsoft.com/en-us/dynamics365/customer-engagement/portals/clear-server-side-cache

https://community.dynamics.com/enterprise/b/dynamics365portalssupport/archive/2017/01/19/portal-troubleshooting-part-one-stale-data-and-bad-requests