flightaware.com/live/flight/ACA1 … /CYYJ/CYYZ
‘Taxiing’ for 18 hours 
, in fact never left the gate.
Just a guess here.
The flight plan was never cancelled, taxiing is being shown because the airplane was powered up when it went to the hangar or different parking spot. ACARS picked up the wheel movement and sent the taxi message.
In fact it never left the gate until about 5 minutes ago to be ferried over to YVR (AC7064). The AC website just says cancelled with no reason, so your scenario may be right - that it went tech. at startup.
Maybe if the FA staff read this they can manually bin the original flight, I guarantee it’s never going to depart 
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ETA: actually it still hasn’t left the gate flightaware.com/live/flight/ACA7 … /CYYJ/CYVR