No: I was wrong to compare my case to dddscy's and have gone back and edited my post. In my case I checked the TWD box manually (and told my server that I was doing so), but did not receive a fresh customer copy. (The 'draft' receipt included the phrase "no signature required".)

I won't "name and shame" the restaurant: dinner was wonderful and I am not accusing them of deliberately attempting to rip us off. It seems more likely that the cashier made a mistake (or was on autopilot and selected the option most often chosen by foreigners?).