Hi again,
thanks for the quick reply!
We just tested different versions and it turns that I was wrong, we did indeed have slight differences in our versions.
The spine file exports correctly from 3.7.17-beta, but came out wrong from 3.7.16.
To show what has happened, I've attached our correct and the buggy export png.
So, for us now, this is solved, and I'll kick my buddy to better not use (older) betas...