Well, I'm not familiar with MIG 21 model, and really I can't tell what should I be seeing at the wingtips...
Just guessing, Western may be right and is an "only one side poly" issue.
Settings in 3dsMax Material editor apply inside max viewports, but don't have any effect in IL-2, so if you select doubleside material property it won't change anything... you need to actually build the two sides with polys.
Like in most 3d engine in Il-2 there is backface culling so you only see one side, the one facing the camera, to be able to see the other you need to put some triangles facing the other side..
But I bet it also has to do with the Symmetry modifier you applied.
Everytime you use Mirror to make symmetric parts, one half gets their axes inverted, and shows with normal faces inside out (the other one, with correct axes orientation shows right) .
To fix this you can apply a ResetXform modifier to the wrong half.
This will fix the inverted axes orientation ...but as a side effect, will make the faces or your mesh get inverted.
Then, to fix them, just select the offending ones and use Flip (inside EditMesh or EditPoly modifier) to restore back their normals.
Be aware that ResetXForm, it also aligns the Local axes with World coordinates so if you have a part with their Local axes rotated (like with moving parts) you will need to rotate them back again...
Is always good idea to apply ResetXform before exporting, but be very careful, especially with animated parts, as after applying ResetXForm changes can't be undone!