Lune I think it's possible that what you mean by "default" is different from what Spine's settings ensure: if you set the default curve to Bézier in the Spine settings, then when no keys are added to the timeline, the curve type of the first key will be Bézier. However, if you add a key to a timeline that already has some keys, the graph will inherit the curve type of the previous key, so if the previous key was linear, the next key will also be linear.
By the way, you mentioned that you are using version 4.1.15, but we recommend that you use the latest 4.1. While major and minor version updates should be done with caution, it is always safe to update patch versions. There are several bug fixes between 4.1.15 and 4.1.24, which is the latest stable release, so you should update your editor version.
