Over the past week or so there have been many users posting in #development-support:building-support about unions that “disappeared” when they published their game. This is a very common problem for those of us who use unions in our builds, and as a result I would like to share with you all how I get around it.
Note: While I know this can probably be explained in a few lines, I am going to expand the directions and include step-by-step pictures just in case new users have trouble figuring it out.
The problem starts here with a union that, when you publish the game, becomes this:
It essentially disappears, but remains in the explorer. No amount of separating can fix this problem. Your union is essentially gone for good.
Here is my solution:
Begin by remaking the union that disappeared. If you have just finished the union and it is complex enough you don’t want to risk its disappearance, just continue.
Once you have the finished product, you are going to export the union into .obj format. You can do this by right-clicking the union in the explorer and choosing “export selection”.
Name it something recognizable and in a place you can easily access.
Then, open the advanced Objects menu, search “MeshPart”, and insert one into your game.

Using the folder icon in the properties panel, upload the obj file of the union you had just exported to the meshpart you have selected.
When you try to upload the mesh there are two windows that may pop up. The first only appears if your union is extremely large, and it asks if you would like to resize it. Click “No” so that the union remains the correct size. The second pop up will ask you if you would like to move your mesh to its given location, click “Yes”. This will ensure your union mesh replaces your disappeared union at its exact location.
That’s it! Now you should have a meshpart with the exact size, shape, and location of your vanished union. Recolor and texture it however you’d like.