When using the Advanced Query from the APFW interface, the Field Name list stops short of listing all of the available fields. The listed fields are listed alphabetically and the list stops in the "Ps". There are hundreds of custom fields in this database. This problem does not occur in the Windows interface for the same database, only the browser interface using either Chrome and IE.
The demo database works fine, but there are no custom fields there of course.
The customer recently upgraded from v19.1 to 21.1, and I do remember that this was a problem with 19.2.