If employees go to a bar after an official workplace social event and have an argument, should the altercation be considered a workplace issue?

Generally, events that are not organised and/or paid for by an employer would not be considered “work events”.  If employees choose to leave an employer-organised event, and go elsewhere, the "event" at the new location could not be said to be employer-event - unless, for example, a manager/senior employee was present and buying drinks with the Company credit card. 

However, if an employee makes a complaint about another employee under the Company's grievance or dignity at work policy in relation to events that occurred outside work (but impact the employee's relationship inside work), the Company may have a duty to investigate, if there is a significant link to the employment.