That has been discussed many times here, and it's because that AI model is not FSX native, but made for FS9. This, in addition of being slower than a native FSX model, has the side effect that will disappear on an airport that uses custom runway textures IF the "Airplane cast shadows" option is ON.
So, you have two possible solutions:
1) Turn the "Airplane cast shadows" option off
OR
2) Upgrade that model with an FSX native version