One think I can think of is that for "offset" to work, the first and last key need to be identical.

then it probably doesn't work for mesh. I also found that offset doesn't for path too. The key will just shift position without the circular update.

As per documentation, the function is not implemented for deform keys: Dopesheet - Spine User Guide: Key Offset

Key Offset requires the first and last keys to be the same and only works on rotate, translate, scale, shear, and slot color keys.

But I've added an issue for this: Make offset work on deform keys · #344 · EsotericSoftware/spine-editor