If all items on a record are marked not for loan on the item or by item type configuration, the result list displays: None available. This is not quite true. Usually in Koha 'available' is used to indicate the item ins on the shelf, 'available for loan' or 'not available for loan' would indicate that the items are there, but cannot be checked out. I think changing it to 'none available for loan' would be more accurate here. Or alternatively: Treat them separately and use "x availble for reference" similar to the result list in the OPAC.
Appears to be fixed in 20.11 :)
Sadly, only the configuration changed. Not for loan doesn't mean unavailable necessarily, reference books are a great example. We should fix this.