That is strange but if they are wired in series, this would make sence.
FYI, I replaced my backup lamps with LED's. The lamp out icon did not come on. I then replaced the running lights with LED's which caused the lamp out indicator to turn on. All the operationally critical lamps are canbus monitored. The brake, running, and directional lights are canbus monitored. The directionals will also hyperflash do to the change in load.