I think it’s the same re-indexing issue from https://forum.inaturalist.org/t/about-the-field-introduced/6091. As an example, Limax maximus currently shows 140 observations when introduced=false is in the search.
If I click on the 140, I can go to a few of the observations listed and manually re-index them, then the search will show fewer introduced=false Limax maximus observations.
You can also see that the majority of Limax maximus observations in Australia come back under introduced=true:
https://www.inaturalist.org/observations?introduced=true&place_id=6744&taxon_id=62470 returns 282 results while https://www.inaturalist.org/observations?introduced=false&place_id=6744&taxon_id=62470 returns 136 results.
If the establishment means weren’t changed recently, then I think this should be reported as a bug.