Strange. I'm aware from personal experience that foreigners can be subject to withholding tax when working as a consultant (with a WP) for a Thai company; paid monthly and withholding tax deducted monthly.
The company had never before employed a foreigner, so they called the Thai Revenue dept., who asked if the foreigner already had a Thai Tax number. The answer was YES, then the Rev. Dept., advised to deduct withholding tax.
Revenue Dept., officer also suggested the company concerned might like to (in the following January) help the foreigner to claim the withholding tax and that's what happened. In fact all very simple and the tax was refunded to me very quickly.