Questions about FMS - SEC F-PLN

[LingYi]

When I try to manually set a sec flight plan, the Airways function seems not to work properly.

According to the flight plan, after enter the airway via W56 to IGMIG, the TMPY F-PLN button pops out.

The airways after waypoint BENBI will be inserted into the active flight plan at some time, or the active flight plan will be influenced for unknown changes.

If clicking the Insert tmpy, the active flight plan will be changed while the sec flight plan will not be influenced. After repeating the operation some times, the SEC F-PLN will lost all waypoints.

I am wondering if the logic above is the way A350 actually works, or a bug.