Rainbows are (very) prone to mycobacterium infections so this may be a possibility
Note that fish with advanced internal parasite infestation (including helminths) generally display an initial (& often enthusiastic) food response, but then consume very little actual food, ofttimes appearing to taste and spit
This is exactly what they have been doing although not spitting out all the food. They get excited as soon as food hits the water, not all the food gets eaten though.