Writing to propcases is a sort of experimental science and a hopeful hack that just happens to work rather well for us... but certainly you can't call any odd or missing behavior a bug when dealing with them.
You can write to any propcase, but the camera only acts on some -- and even then the effect is often partial, usually the GUI isn't aware of your changes even though the all the shooting code is. The wiki documents what people have found over the years. If a propcase appears to change with camera operation, it's meaning has been described. If writing to it seems to have an effect, it's been tagged read&write and if not, it's read only. If it hasn't been tagged, nobody's bothered to check and/or document it yet.
Apparently this one's a read only propcase.
Btw, note that propset is what we call a set of propcases (Canon shuffles the propcase numbers sometimes, we know of 3 sets...and note that they don't actually follow the digic version II/III/IV).
