Odd reverse extrusion (ender3 pro + marlin 2 on skr v1.1)

Hi

I've recently installed a touch-mi probe and enabled the probe and bed-levelling in marlin 2 (latest bug fix).

I'm seeing that the filament is retracted before bed levelling begins but then it is not extruded when it has finished. I think that's ok as per the original gcode as the retraction is 6 and designed to prevent oozing. However when the gcode runs in Marlin2 I see that a multiplier of 51 is added, resulting in the filament being far away from the nozzle and the first layer failing.

extruder calibration is done and verified.

can anyone suggest where to look to fix this?

thanks
Justin