A google search of: "fusion 360 polyline import problem" brings up lots of problems importing (or inserting?) .dxf into Fusion 360, even in the year 2019,
from other programs than MoI.

Another link, Sept. 2018: https://knowledge.autodesk.com/support/fusion-360/learn-explore/caas/sfdcarticles/sfdcarticles/How-to-import-DXF-files-in-Fusion-360.html
states that:
"Issue:
You would like to insert a DXF file into Fusion 360.
Causes:
DXF files cannot be directly opened in Fusion 360, they must either be inserted into Fusion 360 using the Insert > Insert DXF tool in the Model Workspace or uploaded through the data panel:"
(There is more information...)


- Brian

Link, re using Online Data Panel of Fusion 360:
https://knowledge.autodesk.com/support/fusion-360/learn-explore/caas/sfdcarticles/sfdcarticles/How-to-import-or-open-a-file-in-Autodesk-Fusion-360.html

Scaling:
https://knowledge.autodesk.com/support/fusion-360/learn-explore/caas/sfdcarticles/sfdcarticles/Imported-DXF-file-in-Fusion-360-is-not-to-the-correct-scale.html