Follow-up: For some reason, when I rebuilt the portal and included the Constant field as one of the fields displayed in the portal, the issue fixed itself. Still works after removing that field from the portal. Strange.
It might have needed to be indexed to make the relationship valid. Showing it once, I bet did that "indexing" for you. In the future, if you need a constant for a relationship, be sure to set the storage to Automatically create index (and set to "All" wouldn't hurt, because it's probably a calculation that is used in all records and does not change after intial set up.
There is a field in all my tables, called "constant," which auto-completes the calculated result of 1. I use this to join tables when it is not important to match specific keys.
This method has been obsolete since version 7. Use the x relational operator instead, with any two fields serving as matchfields.