More the other way round. In a custom loop you have some open areas in the reservoir where evaporation could happen, you don't really have that in a closed loops, it doesn't just evaporate through the tubing. Techincally it is possible yes, but the amount of evaporation is so small that it doesn't really matter.
I don't own an AiO, but not that I know of. Considering Asetek uses the same system in their Enterprise Server line that would be pretty weird.