Is ㄙㄨㄚˋ (sua4) for the character 嘴 from another Chinese dialect or language? In my explorations of my area of Taichung today I noticed this sign with Zhuyin Fuhao and took a photo to look it up when I got home:


And it seems that ㄙㄨㄚˋ (sua4) is not even an existing syllable in Mandarin, let alone a pronunciation of 嘴, which is pronounced "zui3".

Sometimes a Taiwanese (Hokkien, Minnan) pronunciation is used in some contexts here instead of the Mandarin one, but in this case it doesn't match the Taiwanese or even the Hakka or any other Chinese variety I can find.

Does anybody have a theory where this sound might originate?

Perhaps a Taiwanese word that people don't know the character for, and they just used it with this other character for some reason?
紲喙 (suà-tshuì)
  
  合口味。
  Palatable
  
  形容東西合胃口、好吃，一口接一口，愈吃愈想吃。
  The term describes that the food is so delicious that you want to eat more.




紲 is a substitute character (替代字) and means "continue".



source: 臺灣閩南語常用詞辭典