Repro steps:
Using the UIElements Mesh API...
a) Submit the following three vertices: (0, 0), (100, 0) and (100, 100).
b) Submit the following indices: 0, 1, 2

Expected result: a triangle should appear
Actual result: nothing appears

Submitting them in counterclockwise order makes it visible.

See the example below:
https://forum.unity.com/threads/draw-a-line-from-a-to-b.698618/#post-5080004