I TRIED TO USE FLIP NORMALS AND I STILL HAVE THIS PROPLEM
This is a Z-sort issue, caused by objects having transparency, and the game engine not quite knowing how to deal with it.
In the material settings, objects should be set to the first one that gives the result you want:
- Clip Alpha
- Alpha Antialiasing
- Alpha Blend or Alpha Sort
ie: if you have something that does not need transparency, make it opaque. If you have something that only has clear bits and opaque bits, make it clip alpha, and so on down the list, each giving a slighly more 'advanced' but more processor intensive alpha.