It is because in that result Ret/DD for the first run is 1553 - there are only 7 trades, and Net profit OOS = 500 while Drawdown OOS = 0.5This creates very big Ret/DD value which then causes this spike.
Possible solution - in the WalkForwardStability computation ignore the period value that is bigger that 3* stvdev of all values.
This will remove "extreme" values that disort the result.