This all comes down to X-Plane's VRAM management system. It believes you are running so low on VRAM that it has to make all textures lower resolution. Assuming you have reasonable amounts of VRAM on your GPU (at least 4 GB), you can/should report this as a bug to Laminar Research. The next version (V1.8) is also going to attempt to workaround this, but there's no estimate for when that will be released. In the mean time you can try reducing the texture quality setting so your VRAM is not under as much pressure.