opSplitPart not behaving like UI split part

Hi,

Referring to the image below:

I want to split the part with the surface to give inner and outer parts.

I'm using

opSplitPart(context, id + "splitPart1", {
"targets" : targetPart,
"tool" : qCreatedBy(id + "extrudeOuterEdges", EntityType.FACE),
"keepTools" : true
});

As you can see at the front left, the split is splitting the part full length below the surface giving me an upper and lower part, rather than the required central and outer parts.

If I manually split the part using the surface it works absolutely fine.

Any thoughts appreciated.

Thank you.