I’ve encountered a minor defect in the drag and drop processing on the Toolpath dialog (Windows 10 64 Bit).
If you drag a toolpath DIRECTLY over another, and drop it, the toolpath disappears.
Drag bottom TP directly over the fourth
And Drop. The fifthTP disappears.
Repeating the process until there’s only one TP left
HOWEVER: This is only a render issue. Saved the file as Test and then opened the project…all the toolpaths are back: