Discourse have fixed the underscore bug in their master but I don’t think they have released a new binary for @ChrisGammell to update this forum with yet
Workaround: click on a via as the starting point or start the trace from the other direction. (My guess is that KiCad gets confused as both sides have the same net)
You could report this as an issue on the bugtracker.
If you have a functional issue with using any application be it the project launcher (KiCad), schematic editor (Eeschema), pcb editor (Pcbnew), Gerber viewer (gerbview) and other executables.