It is common practise, not to screw down the pan anymore.
Fears of hitting pipes, wire and cracking tiles are the most common reason for it, but this those not stop the plumber from using alternative mean of fixing( the most common one being silicone) it will be also the easiest one to remove if you need to have access .
How far of the wall is your cistern? As he silicon the pan down?
My solution asking him to put silicone to the back of the cistern.
My Bespoke Bathroom Ltd.