What's up with low vowels in Sabah?

Sabah, one of the thirteen states in Malaysia, is a political region of the island of Borneo. All of the indigenous languages of Sabah are Austronesian, which are divided into four genetically distinct subgroups: North Borneo, Barito, Malayic and Bisayan. The languages of Sabah are likely to have been subject to extensive contact influence, which would make Sabah a linguistic area in the same sense as Melanesia or the Balkans. Evidence that this section of Borneo is a valid linguistic area is given by cross-linguistically comparing the constrained distribution and interrelationship of the two low vowels, [-back] "/a/" and [+back] "/o/". A comprehensive review of the available literature finds that, particularly in the languages of north and west Sabah, a syllable with /o/ can never directly precede a syllable with /a/; nor may a syllable with /a/ directly precede a stressed syllable which isn't also /a/. The main strategy for avoiding these scenarios have typically been labelled as "vowel harmony", where either /a/ will change into /o/ or vice versa - naturally, this analysis finds that the term "vowel harmony" is inaccurate for describing these phenomena. Similar constraints of this sort on low vowels are not found in other parts of Borneo or the neighbouring islands of the southern Philippines, nor are they restricted to one particularly subgroup of languages in the region of Sabah - hence this is a contact-induced areal phenomenon.