I hadn’t got around to complaining to Taishin/Richart yet, but funnily enough I just received another withholding statement for the NT$300 account opening bonus, and in this case I was listed as “ZZ” (not “GB” like last time).
I’m still listed as non-resident, but no tax was withheld. IIRC, this is in contrast to what @slawa reported for his account opening bonus.