I think it’s because you’re using the common names in the US. “Douglas Fir” is set as the preferred name for the US, while “common Douglas Fir” is set as the global default (https://www.inaturalist.org/taxa/48256/names).
So it looks like the tab title pulls from the global default, but the page is behaving as intended and showing you the chosen local (to the US) common name. This was the same situation in the issue with https://forum.inaturalist.org/t/page-title-on-observation-detail-page-does-not-follow-local-common-name-preferences/19351/4