I've not read that in my handbook but will look, certainly no notice on our windows.
I always close blinds (even the skylight ones) when the caravan is in storage to decrease the heat and fade. We had cane conservatory furniture and the cushions disintegrated when I moved them, I guess from the sunlight.
We have a Bailey and we tend to pull the curtains when it's really hot outside to keep it cool inside. The only reason I can think for half up/down blinds is maybe to keep the air flowing. I did read that the windows can get really hot if the blinds are down.
Pull the curtains to save fabrics and woods from fading and keep the van cool. If pull the blinds down, even half way, long term it can damage them. the reason for stating to pull them only half way down, is apparently if you pull them fully down in the sun the heat builds up and supposedly can damage/blow up kind of the window/sun roof behind.
I can't find this in our manual for our new Pursuit. However they do show two types of blinds as the manual also covers Unicorns.
I guess you have the Remis blinds with sunscreen in the bottom and blind in the top. Having them halfway will presumable equalise the tension on both cassettes.