the title of the thread is a little misleading. the issue you’re seeing is not related to observations. it’s more related to taxa in the checklist.
here’s a thread that probably has the most comprehensive existing discussion on the topic: https://forum.inaturalist.org/t/place-checklist-doesnt-show-all-research-grade-species-observed-in-that-place/11609. it takes maybe 5 minutes to skim through, but it provides a lot of context, and this post also provides some additional helpful discussion for some situations: https://forum.inaturalist.org/t/populating-project-location-checklist-with-older-rg-records/18558/3.