I’m pretty sure this is leftover from the bug where changing the establishment means didn’t automatically re-index observations. That’s been fixed, but the fix wasn’t retroactive.
https://forum.inaturalist.org/t/how-to-use-inaturalists-search-urls-wiki-part-1-of-2/63/320
I re-indexed the observations that were causing problems for your particular search, but if you change the search parameters, for example by adding May, you’ll get more introduced species.