Hello Goku.
I tried to repeat this situation without any success.
Impossible. Perhaps I made a mistake...
However, I noted a particular situation.
1.You just layied some tracks and you are in Object mode.
2. You remain with SELECTION selected
3. In Menu Tools, You select TERRAIN MODE
4. then you can move tracks in all directions. The DBLine remaing alone.
Thank for your attention. Sorry for your time lost.