Can you open your file (created by the Spine 3.3.07) in the older version (Spine 3.2.01) ?
No, I can’t
Did this happen when simply saving the file from one version (3.2.x) to another (3.3.x)?
Yes, just re-save old project in new version.
At what version did this happen?
Or have you started using any of the 3.0 features?
We use latest spine. But my animator told that he don’t use any new features, just new version of animation tool for working without some bugs.