%ExistsId and %OpenId methods for linked tables in FHIR Facade do not work

My IRIS instance is connected to a Postgres database using SQL Gateway and linked tables.
One of these tables is projected to the Patient class. I want to select a record from this table by ID and convert it to a FHIR resource using the %ExistsId and %OpenId methods.
I noticed that if I call these two methods from the console, the record is always found.
But, if I do the same from the FHIR Facade layer, the %OpenId method returns NULL. The error is intermittent - yesterday FHIR Facade didn't work at first, then it worked, this morning when I try to select a patient record from the FHIR facade layer using the %OpenId method it returns NULL again. I also checked the context of the current user using the $USERNAME function, in both cases the current user is _SYSTEM. How can this be explained?