I'm not sure this counts as a bug: the Sine behavior uses the size of the object set in the layout which is positive 128. So it initialises like this:
Create Arrow size 128x128
Initialise Sine behavior, width = 128
Start of layout: set width -128
After 1 second: activate Sine behavior using saved width of 128
I could change it so activating the Sine behavior uses the value at the time of activation, but I feel it's likely this could break existing projects that deactivate and reactivate to "pause" the behavior. So closing as won't fix for now - easy workaround is to set a negative width in the layout view instead of setting on start of layout. Let me know if there's anything else you think should be done about this.