In the test case, the NavLink component has a navigation cost of 1000000000. When the AI navigates to the NavLink the printed cost is only 2677. It is believed that the AreaClass isn't being taken into consideration and is registering as a default value of 0. The path cost using the ramp instead of the NavLink is 4475. Which if the high AreaCost was being applied would be the logical route for the AI to take instead of the NavLink.
This issue was reported and tested in 4.20.2 (CL-4302132). It was reproduced in 4.19.2 (CL-4033788) and Main 4.21 (CL-4347190)
Results: The NavLink components Area Class default navigation cost isn't added to the Path Cost of the AI's Navigation Path
Expected: The Area Class's default navigation cost is added to the path using the NavLink
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-63808 in the post.