Try just committing records first. As a test, you can click the layout background after changing the category. If this works, you can use a script trigger and the commit records script to automate the update.
That doesn't always work so there are other options.
If you have FileMaker 15, try giving the portal an object name and use Refresh Portal in a script to see if that works.
Then try "goosing the match field" if none of the above works. In this, you use set field to set a match field in the layout's table to itself to force any portals based on that relationship to update. But be careful. If any look ups or other auto-enter options use that relationship to look up information from a related table, they can be updated by this step as well.
Thank you for the quick response. Clicking on the layout background does seem to solve the problem. Very curious. So you would suggest a script that commits the record and which triggers when a category selection is made from the dropdown?
Also, yes I am using FileMaker 15
If your value list allows the user to type text directly into the field, use OnObjectSave. If it does not, use OnObjectModify.