Dear Howard,
While I do not have an "explanation of the status," note that the Isa and Brhad-Aranyaka share much more than the mantra: several of the Isa verses appear (with some variation) in the 4th book of the Brhad-Aranyaka (Yajnavalkya's teaching to Janaka, specifically 4.4.8-21). Also, the last four verses of the Isa that begin with hiranmayena patrena etc. are Brhad-Aranyaka 5.15.1, thus the concluding portion of book 5. Note also that both belong to the White Yajurveda and that the Isa is the last chapter of the Vajasaneyi Samhita (Vajasaneya being a patronymic of Yajnavalkya).
All best,
Aleksandar