Cistern refilling 'delayed'

[Old UKPF 96064]

I have a Villeroy & Bosch cistern with a Geberit dual push button valve - about 15 years old.

After flushing, the cistern refills to about halfway and stops. Then, the valve seems to randomly allow a bit more water through. There's no indication that the cistern gets more than about 2/3 full and there's no evidence of overflow. I'm just curious as to why it does this.

Any ideas please?

Thanks

 

[rpm]

The push button valve allows water out of the cistern another valve controls the water coming in. After 15yrs the float valve is sticking or the adjuster for the water level cut off has moved?