I'm using Spine version 4.3.37.

Open the Spineboy project.

Select any mesh, then choose any loop edge.

Hold Shift and click on a vertex to quickly select the entire edge loop.

Now, using the Rotation tool in Setup mode, rotate the selected vertices — this behaves as expected.

However, if you switch to Animate mode while keeping the selection and rotate the same vertices again, you’ll notice that one of the points behaves incorrectly.