Blust (1977) had argued that a politeness shift changing the PAN 2pl pronoun *-mu to 2sg is a PMP innovation. Sagart (2004) proposed that the Buyang 2sg pronoun ma312 is cognate with PAN *-mu 2pl and PMP *-mu 2sg, thus sharing the PMP plural-to-singular shift. Blust (2014:375) states that u::a is not a recurring correspondence between AN and Buyang, urging caution. The correspondence in fact recurs in the other second-person pronoun, PAN *iSu 2sg, Buyang ɕa54. The Buyang form occurs in the plural form tsum54 ɕa54 2pl, (tsum54 plural marker of personal pronouns). While the habitual fate of PAN *u is to Buyang u, the fact that it evolves to Buyang a in both 2nd-person pronouns shows we are not dealing with a chance resemblance. The politeness shift to singular of the PAN 2pl pronoun is shared by ̴PMP, Buyang and, to my knowedge, the rest of KD. While the evolution of vowels in 2nd-person pronouns is complex, in KD languages where a 2nd-person pronoun has the phonetic shape /mV/, that pronoun is singular unless accompanied by a plural marker.
Blust, Robert A. (1977) The Proto-Austronesian pronouns and Austronesian subgrouping: a preliminary report. University of Hawai’i working papers in Linguistics 9, 2: 1-15.
Sagart, L. (2004) The higher phylogeny of Austronesian and the position of Tai-Kadai. Oceanic Linguistics 43,2: 411-444.