Here's a strange effect I observed on two retina Macs (iMac, MacBook Pro) natively running Elementary OS Loki (i.e. Ubuntu 16.04, no vm involved): https://www.dropbox.com/s/xna6kv4vdqi4ken/krita-retina.webm?dl=0 Coming from the bottom, the cursor will stop at a certain line. However, if you're already drawing, this won't happen. Also, if you come from the top, things work fine. Also, this only happens with a Wacom pen, not with a mouse. I suspect this is related to the retina screens. It doesn't happen on a regular Linux machine I work on.
I would rather suspect your xwacom setup -- this isn't something we don't really influence at all.
We cannot do anything about this.