If you edit the card text after that remap, you can re-populate the Taiwanese Mandarin field with the correct pronunciation. The example seems to be a link to another card, if you tap on that 'outgoing links' item you'll see a screen for that card where you can likewise put the correct pronunciation in the Taiwanese Mandarin field.
We've been going back and forth about whether to have Taiwanese Mandarin auto-populate like the other Mandarin field does; the basic problem is that since we have a lot fewer entries/dictionaries to draw from in generating those readings, in some cases we may end up with readings that are less accurate than what we get now by simply using the data in Mandarin where there's no separate Taiwanese reading.