It happens all the time on CX First Class it does not mean they are oversold.
There are only 6 seats in the Cabin
2 are blocked for CX Elites
2 are blocked for People with babies
So if you are not one of the first 2 to book it shows no availability.
You can with phone pressure get CX to release an elite seat but the 2 Bassinets Seats 2A and 2K are usually only controlled by the Airport.