CYa does not evaporate, it will concentrate in the water as water evaporates (the same as any other ionic solid in the water, i.e. calcium, salt, etc.).
HIgher CYA just means you have to run higher FC to compensate (to keep enough 'active' chlorine in the water and not bound up with the CYA as a chlorinated isocyanurate) unless you decide to do a partial drain and refill.
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