Basically, all income you receive for work performed in Taiwan is considered “Taiwan-sourced income”.
For more information, see:
In short: If the company is a legal entity, dividend payments might count as “overseas income” and might be tax-free in Taiwan (but you’ll most likely be taxed at source). Salary payments, on the other hand, would count as “taiwan-sourced income” if you work from Taiwan.
Also, the tax authorities could demand that the company (even if overseas) files a tax return in Taiwan for the income they earned in Taiwan (e.g. through the labor of Taiwan-based employees). And there are some laws against overseas shell companies which could be considered as Taiwanese companies in some situations. In practice, I am not sure if the tax authorities would actually follow through on these two issues especially for small to medium companies.
As always, seek professional advice when in doubt.