Is NB-IoT roaming possible?
If you use your favorite search engine and search for terms like "nb iot roaming" you'll find plenty of other pages backing this. 

I'd interpret this (2017) document from the GSMA in a way that it says "NB IoT specifations have been written very quickly, Roaming might be added later" 
https://www.gsma.com/iot/wp-content/uploads/2017/08/CLP.28-v1.0.pdf

This source https://1ot.mobi/blog/the-state-of-roaming-narrowband-iot-lte-cat-m1
says


  There is a small beam of light indeed. Deutsche Telekom and Vodafone
  completed the first successful NB-IoT Roaming trial this summer.
  (this seems to have been carried out in 2018) 


To answer your question:
Yes this seems to be (have been) a technical limitations per specification, but people are working on it.