Wave, I have no issues with robbing while feeding sub pollen.
For years, I feed sub pollen, Ultra Bee. I have two open, community feeders in my apiary, between hives and have not had any issues with robbing. One feeder is only six feet from a healthy hive and no robbing issues with the other 21 hives.
I agree with others, if pollen is available, the bees will ignore the sub pollen. My bees appear to delight in the Ultra Bee, the bees roll in the stuff as Mann Lake places lemon grass oil in the pollen and the bees love it.
I have made my own pollen feeders, raccoon, possum, water proof, 4 inche plastic pipe with gutter end suspended by wire. I place sticks in the pollen powder so the bees don?t sink in the powder.
Best to the bees.