It would not certainly not automatically be a reason to claim a CE under (19). It could be that the Contractor had not done their homework on them with credit checks and things like that, and therefore why should an Employer have to take on the liability of the Contractors poor choice of subcontractor?
Having said that there is nothing to stop the Contractor from notifying it as a CE and seeing if they get any sympathy from the Employer. There could be circumstances where they are more sympathetic – e.g. if the Employer “nominated” the subcontractor, or if the subcontractor was the only one the Contractor could really have gone to.
Hope this makes sense. To summarize – default position would be “no not a CE”, unless there are circumstances that make this more of the Employers liability by limiting the Contractors choice of supply chain.