I am experiencing a problem, after a certain number or brushes I made in my level, every other brush I would place were becoming invisible and making some faces of near brushes invisible too. I selected a part of my level and made as another level to see if it was the number of brushes I had placed in my persistent level but the problem continued and to fix it I would need to rebuild geometry to all affected brushes would became normal again.
how to solve this problem?
Investigation update 1:
I’ve deleted some brushes and created a new brush but the problem persists so actors count isn’t the problem.
I’ve noticed that in the same affected brush faces some parts are rendering, others still become invisible.
Brushes can be set to Additive and Subtractive, where the second one will make them look invisible and will Subtract from Brushes within them.
I suspect that you’ve accidentally hit a Hot key and made them Subtractive, OR there just might be a bug where they are becoming subtractive on their own.
Either way, if it happens again, check the details panel and see if it gets fixed if you change it to Additive
This was my first tough but all brushes are Additive.
I am having the same issue.
Everything was going really well at first, and I would copy and paste the brushes and link them together.
Then my brushes started getting “sections cut out of them” (not sure how else to explain this) which the player can see through during game play. It is also doing it in the viewport.
I have tried changing the order of the brushes which sometimes fixes some of them, but almost always breaks another pair. It’s gotten so bad that my floor is actually getting cut out in certain areas as well.
I am using additive brushes for the boxes and pipes and then that same shape as a subtractive brush to make them hollow. I know there is a hollow option but then the brushes cannot join / intersect one another which I need them to do. For example, a pipe must lead into a box and the player needs to be able to walk inside them, NOT through them.
I am also able to see through some of the brushes from a specific side. Please help! 
I don’t know why but, I found that if you separate the brushes in streaming levels they work normal again. For example, I made a streaming level for every floor of my building and it solved the problem.
Its a work around but as far as no one being able to tell why this happens its my only choice. But several dudes told me to always work with static meshes, so after finishing my building I will learn 3d modelling and create static meshes of the group of brushes I made.
Thanks so much. I will give this a shot.
