Placing player built walls doubles vertically in some areas, but not all!

I am building a box fight map and in some areas (at the edge of the arena) player builds are doubling vertically. It only uses the default 10 mats to place both walls. Building within the confines of the arena is perfectly fine, the player builds do no double or exceed the mutator zone space. Doubled player builds on the edges happens with and without a mutator zone present.

Thank you for your report! We would like to look into this further, would you be able to submit a bug report using the form available here? Fortnite Creative