Maybe a late-Ming era massacre so thorough (even worse than OTL Sichuan) that large parts of formerly Han land are reclaimed by indigenous minorities that then get absorbed by the Zhuang. The major cities remain Han until industrialization, when the Zhuang rural population floods into the cities.
Hainan has virtually no Zhuang population in the first place (its indigenous people are Hlai) and the Han Chinese are far too entrenched in the Pearl River region. Any Manchu/Mongol/RT/etc massacre will also affect the Zhuang. Even if it doesn't, the Han Chinese will come back because the state will support them.