why is 'will leave' incorrect?
Because it has a slightly different meaning:
Ah thanks again Joanne - we haven't covered falbhaidh yet. I'm feeling on a bit of a learning plateau just now - is there fresh content in the pipeline? Also, any tips for getting some conversational practice in Glasgow area?
Màiri is leaving in a minute should also be marked as correct because it is an acceptable way of referring to the future in English.