Checking of search() result for reference cursors before join()

Hi!
I have next question - if i use in join cursor reference cursor with one of compare condition "gt", "ge", "lt", "le" (not "eq"), should i check that result of search() call for reference cursor is 0 or it can be WT_NOTFOUND too? It looks like sometimes join works fine even if result of search() for reference cursor was WT_NOTFOUND. I attached small program that shows cases in which that is not so.