Andy,

When you GOSUB from a high priority interrupt handler by using @ ON_USBRX_GOSUB ... all interrupts stop.
USB is no longer being serviced. Nothing can be transmitted or received.

When you hang around blinking LED's and PAUSING for several seconds, the USB connection will be dropped. Actually it will be dropped much sooner than that.

You might get away with it if tick_timer: was a low priority interrupt, but certainly not the way you have it.