Hi Mark, looks like it may be a bug, but it also seems to be related to the pretty small size of the object, it looks like your tube has a radius of 0.01 units? That's pretty small in size and is getting close to the default accuracy of 0.001 units. Things can start to behave a bit odd when you have features of the model that approach too closely to the fitting tolerance such as this.

Increasing the size of the object seems to help avoid the problem - I've attached a version of your model scaled up by 10 times so it is not quite so tiny in scale and this seems to behave better.

To do the cut, select the spring in the attached file, then run Construct > Boolean > Difference, and select the plane as the cutting object. That will slice the spring into 2 solid pieces and you can then discard the top piece.

- Michael