What I mean is if the EPSG:3857 result is equal to Nextspace, they are the one that’s wrong, and you were correct in using UTM. Then you should find another reference software to calculate the info from OSM data, to validate your own program.
Unless you provide more details, share your own code snippet, source code, etc, we can only play a guessing game on what’s wrong. Or do you already have a public Git repo that we can look at?
Do you have a technical support plan by subscribing to Nextspace? Have you asked them for help?