I can see the problem here. If a flight is delayed until the next day, a new number will always be assigned because you can't have 2 flights departing on the same day with the same number.
However- because of the way that AC868 is normally a day flight, the delay of the one flight to a departure on the same day resulted in 2 flights arriving on the next day - at the same airport - with the same number (now - around 10 hours apart but still - same day). Thus, people looking on the arrivals side, would have had 2 flights with the same number arriving on the same day.
The problem will be that the scheduled arrival date was actually the day before - and you would have had to look at the previous day's arrivals, and see AC868 was delayed until T+1 in order to find the correct information. For the masses of this world - they would have never figured it out. As for the baggage belt identifier with the "A" assigned - that was probably just a baggage system flight number as flight numbers never operate or are published with letters assigned.
It's not so normal to assign a new flight number to a delayed flight when the departure is the same day - but - perhaps that could be something AC could look at - especially for AC868 - mainly because of the arrival time and the mess that can cause.
This was a bit of an anomaly but you're right - AC created some confusion on the arriving end by not changing the flight number and perhaps should have done so right away.