To avoid confusion and misunderstandings about NHI payment, is it easy to check if you have an obligation to pay, or not? Where do you ask? I assume there is some city hall office that takes care of NHI related inquiries?
This becomes particularly important for the case where you have HHR and qualify for NHI, then leave for a long time and suspend NHI, but then return back to Taiwan for just one day or one week. In that case, you are apparently automatically billed for at least 3 months of NHI coverage, as discussed here: Well, that's a stupid rule--Taiwan's health insurance