Playing basic Civ6 with Victoria. Archeologist retrieved 3 artefacts, which were installed in a museum. Meanwhile, I traded 3 more artefacts with gold and luxuries. These filled up the museum (remember Victoria has 6 slots in each museum). When the archeologist retrieved the next artefact, it was shown on the screen, but was NOT put in any museum or palace etc, despite there being spaces available. It simply disappeared, along with the archeologist.