Over the last year, I've noticed an unusual pattern with all three models under the Ultimate/Zibo family. The problem occurs when flying standard routing with the aircraft(s) operating in LNAV mode.
Any OS (Windows 10 Pro 1803 demonstrated)
Ultimate 737-700U (all versions)
Zibo 737-800X (All versions)
Ultimate 737-900U (All versions)
Tested on both a clean install, and a "daily-driver" install.
Aircraft(s) do not maintain a fixed TRK when flying between fixes, causing the plane to fly a bowed pattern either to the left of TRK, or to the right of TRK, depending on direction of flight. The longer the distance is between legs, the wider the plane flies this bowed pattern.
If when flying over a fix, the pilot initiates a Direct-To to the next fix, the plane will fly 100% on the published TRK without incorrectly deviating.
The problem is easily reproducible to the pilot without taking any steps to reproduce. It is a passive issue.
I will attach screenshots below from inside the aircraft looking at the ND, and moving map screenshots from both Little Navmap, and the built-in X-Plane 11 map. For further comparison, I will also provide similar screenshots when flown with other aircraft.
NOTE: Both Little Navmap, and the X-Plane map are using proper globe projection, this is NOT a visual issue