When chia harvester start to check plots for proofs and how often?

I have another one regarding challenges which I posted here. What is the challenge and proof mechanism in a multi-harvester network

I have 3 harvester-plotters, connected to a main farmer.

I see the same challenge hash repeat 64 times on the GUI and all 3 harvesters receive it.
What I dont understand that shouldnt this challenge hash be different every time?
Only once did I see multiple callenge hashes appear in the gui and I could see all 3 harvesters receiving the different ones. Doesnt that mean that a sing challenge hash is not proofed against all the collective sum of plots on all harvesters?

I think renewal of challenge hash after 64 count is a bottle neck at my end or this perfectly normal?