Invisible faces after export

Greetings! i’m having a issue which i dont understand.

Im working with a team on a small game project and i need to export my works from blender to unreal.

So far so good exporting with .Fbx but then one of my mesh got a problem.
When it is imported on unreal engine the hole bottom faces dissapear and i tried joining both objects and separate them but nothing.

Is there a way to fix it and prevent it on future projects?