To answer my own question: you used a decoder that you found in a topic about a different device, the TTGO T-Beam.
True, and very much as expected. And, you’ll get the very same result when using that payload to test the decoder function:
If you still have a question then please be more specific. Note that you can also TTN Mapper with a device that does not have a GPS module, if you use its mobile app to determine the coordinates.