The 2018 arXiv preprint The New Horizons Kuiper Belt Extended Mission by the New Horizons team says in section 6, pp. 21-22:
Nevertheless, throughout the 2020s, there is significant potential for valuable planetary science (and even astrophysics), with New Horizons. Models indicate some 7-10 kg of usable hydrazine (not including the estimated trapped, ~2 kg) will remain at the end of the first extended mission to facilitate unique observations.
Question: Why does New Horizons have ~2 kg of unusable "trapped" hydrazine? Where is it trapped?