Reinstalling won't change how this works, sorry. You can adjust your curves as normal after offset.
Assuming there isn't a bug in 3.8.99, and I don't think there is, when you offset Spine "splits" the curves to keep the same interpolation. The 3.8 graph makes it difficult to see what is going on, so the curves look strange, but they are correct. Make a simple animation that moves a bone, set curves, then offset. You'll see the motion stays the same, even if they curves look strange.
