Not sure if this is an actual fix but it seems a potential solution or atleast a way to up your percentage a creative replay will actually render the props and landscape from your UEFN session.
TLDR: After the island is first loaded in (Clicking launch session in UEFN), and the game starts, that replay will contain your character loading in through the creative lobby, and thus will not load your actual island’s props and landscapes.
If you are planning on making a trailer for an island that is not published (I dont think this is an issue if you join from discover, only clicking launch session from UEFN and it loading into your creative session) , click “end game” for that intial game that automatically starts and start a new one. The new replay will not contain the creative lobby and should properly load in all your assets.
Hope this helps save some headaches from realizing your 10 minutes of running around your map are actually not usable.