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Please see the official X‑Plane Support page for help.
It sounds like you are already familiar with using WED, so to flatten an individual airport you open it in WED ( I use V1.51) click on the name of the airport in the right hand "pane", and check the box that says "always flatten". This will flatten ONLY this airport (assuming you have created the airport boundary) and the other global airports will remain unchanged.
Reply in comments if this helps. and see the screenshot to make it easier to understand. Matt
Officially I am not able to provide an answer to your problem.
However the following link https://www.youtube.com/watch?v=aLd9k6krwUU may be of some assistance.
Hope this helps
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