Uplink port number changes

I fired up a MKRWAN1300 today and started experimenting with the example code. One thing I have noticed is that the port number changes from time to time. I've seen uplinks using port number 2, 4 and 6. Looking further into this, I cannot find the API for setting the port number.

Has anyone else observed this behaviour?

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Resolved the issue with undocumented function modem.setPort();