Yes, that is the problem. The URL parameter is querying the establishment means for each observation taxon in the place checklist where it occurs, not the Observation field: Introduced. Place checklists and observation fields are not connected to each other. See this forum topic for more information about place checklists.
Anyone can change establishment means settings in place checklists, so a word of caution:
Changing a species to Introduced for a large place will make it Introduced for ALL included smaller places. So for example, don’t change a species to Introduced in the Mexico checklist unless it is Introduced everywhere in Mexico. If it is Native to even one small part of Mexico, then it should be set to Native in the Mexico checklist also, and Introduced status should be handled in lower-level place checklists instead, like for states where all occurrences are Introduced.