It isn't a bug. It happens when you weight vertices, then adjust bones so the vertices move. After doing that, each bone wants the vertices in a different position and the weights determine the final position. As a result, when you change the weights, the vertices move.
You can use Update Bindings to rebind the bones to the mesh using the current vertex positions. After doing that, you'll be able to adjust weights without the vertices moving (at least until you adjust bones again and deform the mesh!). Please see the docs here, which explain it in more detail:
Weights - Spine User Guide: Update bindings
