LT3845 not giving 1.3V
In the datasheet, the minimum start voltage (see page 3, second parameter) is listed as 7.5V max.

There is also this note:
Note 4: VIN voltages below the start-up threshold (7.5V) are only supported when the VCC is externally driven above 6.5V.

As you are not satisfying the startup voltage requirement with your 6V source, I do not expect your circuit to properly simulate (or operate, should you build it this way).

Either supply a minimum Vin of 7.5V, or externally drive Vcc (pin 13) with at least 6.5V.

You could always use a different converter, of course.