Are all of the lines drawn being treated by OneNote as "Drawing" (as opposed
ot Handwriting)?
I have found that the following settings helped me keep my drawings separate
from my handwriting, and allowed my to keep strokes of a particular drawing
together:
1) Turn on Ink Groups (View | Show Ink Groups) and make their display shade
fairly dark (Tools | Options | Display | "Adjust the darkness..."). This
will let you see what OneNote considers a single group of strokes.
2) Get into the habit of manually switching between Handwriting and Drawing
*before* writing on the screen (Tools | Pen Mode | Select one of the "Only"
options, not the "Both" option).
3) When creating a drawing, start it as a new Ink group by starting to draw
outside of the previous grey area. Keep subsequent strokes inside of that
Ink Group. For your table example, start with the large rectangle that is
the outside border of the table, then the row/column separators should begin
and end inside of the rectangle. Starting the lines outside of the Ink Group
sometimes makes OneNote think you want to create a new Ink Group.