Reproduction steps:
1. Download and and open the latest attached project (fogfiles link).
2. Open scene "TreeScene"
3. Look at the Gameobjects "DrawSpeedTree_LOD1" in the scene view, drawn with with DrawSpeedTree
4. Open the same project with 5.5+
5. Look at the same objects.
6. Notice how Meshes look
Added video of the problem in the last edit

Expected result: tree mesh is drawn normally
Actual result: the selected mesh is not drawn normally

Reproduced invalid import from 5.4.1p3 to 5.5.0b7, 5.4.1p3 to 5,6.0a1
Not reproduced when importing back and forth on versions 5.4.1p3-->5.3.6f1, 5.4.1p3-->5.2.3f1

Note: new project added from 838946