Art, I compiled your "old" file. (Didn't notice any significant difference between it and the new one in the area of Poly 1.) I don't have UT TAC. But with stock mesh, I was unable to duplicate your issue. Actually, I'm not clear what you mean by "polys disappear when zoomed in". However, I tried a number of different scenarios and noticed nothing amiss.
What I did notice is that your new airport background does not fully overlay the stock flatten. Consequently, the flattening applied in any position outside your new background but inside the stock flatten will reflect the stock flatten. But, that's probably not relevant to your reported issue.
Since I don't have UT TAC, I can't replicate what you have. But, as I've reported in various forums, a flatten is not always flat. I have on numerous occasions seen "ripples" near the edges of flattens, especially where they bound steeply sloping terrain. While these ripples generally are only a cm. or two in height, they are enough to affect the display of GPs in the vicinity. The effect is that at low angles of incidence, (portions of) the GP disappear. Perhaps this is what you are seeing. If so, I suspect applying a sloping flatten off the end of the runway would change things.
Don