Using @Kalpit_3d 's file from another post. The file contains a box and a polyline cutting curve. The box has isocurves turned off.
test_isocrvs.3dm (2.3 MB)
If “Show isocurves” is checked in Options>General:
Wirecut with the curve (normal to curve, Enter for “through”) results in the upper half getting isocurves shown and the lower half not. Flipping the start/end of the cutting curve does not change this, however interesting is if you invert the curve (red curve in file) then the lower half gets the isocurves.
Extruding the surface through and then using BooleanSplit to split the box results in no isocurves being shown on either part of the split box.
Using the curve with Split from the right view to simply split the box results in isocurves being shown on both split parts.
Anything else that modifies the box - like RoundHole, or even just Explode - shows isocurves on the results.
This inconsistency was already present in V7 and has carried over to V8.