Try making your flatten larger?
Back to the topic.
It doesnt help. I noticed it also on the new created airfield (the whole runway disappears at some angles). I saw few topics about this issue on P3D/SOH/Avsim forums but till now it isnt solved. Looks like a bug in P3D from v3 to v5 in some special situations (nobody know it 😄 ) I suppose.
However I solved it finally.... maybe not solved but fixed with one
trick.
Maybe it helps for you if you noticed the same issue.
I found this topic too
https://www.avsim.com/forums/topic/488783-scenery-disappearing-under-certain-angles-p3dv32/ and the last post was interesting for me, checked and Voila!
You must from ADE add the new aprons (TERMAC) in any place far from the runway (4000-5000m) - it fixes this. You can mask it by buildings ect. It may be very small. From then on, this newly created Apron will disappear also on some angles, but this is not very important as it is not part of the airport in terms of airport level visibility.
For example my "fixes" for 2 disappearing airfields.
For the one I added it below VOR station as background, for the second on dark place of autogen of LC.