When using the pen on a surface to draw on a word document, the drawing 'walks' down the page

I use a Surface Pro 4. 

Today I visited a client site and pulled up our proposal in Word. During the meeting the client had several questions, so with the document in print layout, and with the draw tab selected in the ribbon, I began to draw circles around things I needed to review, and write small notes. 

However, whenever I started to write, or indeed every time I went to draw something, the existing drawing would 'walk' a few pixels down the screen with every pen press. 

I can't really figure out how else to explain it. 

Our proposal had many lines outlaying costs and other issues. Lets say I took the red pen from the 'Draw' tab, and drew a line from the bottom of one section, to the top of another. Then Between those lines I wanted to write "review". 

The first line from bottom to top, worked fine, but when I tried to write 'review', first releasing the pen after typing the letter 'r' would cause the first line to jump down a few pixels. And this repeated with each letter. 

I tried it much further down the document as well to make sure I wasn't overwriting something and confirmed it worked there as well. 

It's the strangest thing. It's almost like the drawing layer doesn't "lock" in place. 

Office is fully updated, as is the surface, but I tried this on a few other documents including a blank one and it did the same thing. 

Any ideas here?