I am in the same situation. I was here from January to August, mostly working in a public junior high school (tax free until next year, but that is irrelevant here, right?).
In August I left, and taught for two months in Bangkok (got conned - beware of "international schools" there - this was like the dodgiest of backstreet buxibans here) so I have just come back and started working for the same company/agent as before, but at a different - now private - high school. (Private schools are taxed, public junior High currently aren't.)
They are saying that since I left and have returned on a new ARC etc, I must pay 20% for the first six months. They do not believe that becuase I have been here for six months since January (as per the stamps in my passport) I should only have 13% (??) withheld.
Does anyone know an authoritive site (preferrably government/official, and if possible in Chinese) which I could point them to, showing that 6 months presence in Taiwan makes one eligible for the regular rate?
I am of course assuming (hoping) that everything I read here, tealit etc applies and I am not an exceptional case. Would appreciate any advice possible.