"I've done thick mash-out decoctions before and have always gone from starch negative (iodine test) to starch positive after returning the thick decoction to the mash tun. Since enyzmes are still in the liquid left behind in the mash tun, and conversion happens quickly >160F, a 10 minute wait (and purposely not reaching mashout temperature for batch sparging) fixes the starch issue."
Read the OP's process and what he is asking... You may have mash out confused with a decoction that is used to hit a rest temp, like a conversion temp. Of course, you would pull a thicker mash and boil it in order to hit the conversion temp. That's part of the process in a decoction. Mash out is done to denature the enzymes and loosen up the extract. The mash out decoction is usually pulled after conversion and returned back to the mash tun to raise the mash temp to 168-170F. Most of the mash liquid is pulled for the mash out decoction. If you read what the OP wrote, you will notice that he said he was boiling after conversion. Also, whether you believe it or not. Anytime mash is boiled, starch bursts. That's a part of the decoction that gives the enzymes in the mash liquid more to work with.... Decoctions are generally fly sparged. Batch sparging, somewhat, defeats the purpose of a grain bed filter. The grain bed from a decoction settles differently than that of an infusion. An infusion bed floats, due to entrapped air in the husk. Boiling during a decoction boils out the entrapped air. Allowing an even filter bed to form, that catches the protein mud on top of the bed. Hence, a cleaner run off. Protein mud and unconverted starch in solution are washed through the toilet braid in a batch sparge, when the bed is stirred up.
