Hi! Sorry for the late answer. I have Surface Pro 4 and have no problems at all writing music in Notion. In fact, I think the experience equals that of writing in Staffpad which I also use. I now use Surface Pro 4 and Staffpad as sketching tool "on the go" and do the heavy lifting on an Asus laptop connected to a Dell Canvas 27" Pen and Touch-screen. For me this is the optimum setup and handwriting music on the Dell Canvas in Notion is so incredibly rewarding. Surface Studio in my opinion is conceptually weird in that you have zero possibility to upgrade the computer, not to mention the price. Yes, I also left Sibelius due to Avids non exsisting interest in customer support. I was a Sibuser since the Finn brothers era but no more..
Don´t know about Surface Pro 5 but it sounds like a setting related to "palm rejection" is amiss. When I rest my pen-holding hand on the screen on the SP4, nothing moves around and handwriting music is just as easy as writing on paper. Erasing can be done by "hatching" across the offending entry with the pen or draw a couple of swirling circles around the entry - lift pen and the circled entry is gone.
At least in my setup..