This is true. It didn't help that I'd lost a bunch of heat because of the headspace in my mashtun. I plan to try it again on a dunkel in the next week or so with my 5 gallon cooler and have some near boiling water at the ready.
I think you want a little liquid in the decoction, but not too much. Obviously you don't want it to be just grains, otherwise, there'd be nothing to boil.
I do around 1 decoction per month and have always used 1 quart grist per pound of grain. Using a water to grist ratio of around 1.5-1.75q/lb is a good for me.
The thing you want to try to do is leave as much water behind in the main mash when pulling your decoctions. This leaves all of your enzymes in the main mash, which is what you want. Also the grain holds alot of the water and as you decoct it, the liquid absorbed by the grain will be released. So what appears to be pretty dry at first will end up being alot more oatmeal-y and watery at the end of the decoction.