Rāmāṇujācārya is a Prābhākara Mīmāṃsaka who has also commented in a Bhāṭṭa garb a Bhāṭṭa work, Pārthasārathimiśra's Nyāyaratnamālā. In Rāmānujācārya's works, Kumārila is constantly referred to as “ācārya” (see TR IV, §3, §3.17, §9.4.1, NR ad AN, III, 28, p. 253). And also Pārthasārathi Miśra uses the same appellation (1937: 75).
Is “ācārya” a self-understood way to refer to Kumārila? Is it common in Buddhist sources, too? Elsewehere, ācārya seems to be used in order to refer to a teacher of a rival school (against guru). So possibly in Madhyamaka literature, see D.S. Ruegg 1981: 58, fn. 171 (where svayūthya is opposed to ācārya).
By the way, R. does not call himself “ācārya”, this appellation is used among Sanskritists chiefly in order to distinguish him from the best known Śrī Rāmāṇuja.