Hi Pharan,
In hindsight I should have posted immediately after seeing this happening as I can't recall the exact version now. It happened recently within the last couple of months.
Sorry I can't be more helpful. I incorrectly assumed that it would be caught and fixed without me having to post but I've had it for at least a few weeks now and thought I'd post since it's not fixed yet.
It's there on first launch but (after doing a little more testing), either method of resizing the window via dragging the bounds or using the fullscreen mode toggle button, will fix it.
I'm on Spine 3.4.02 at the moment with OSX sierra 10.12 (maybe it's the version of OSX that has introduced the issue?)
Thanks