I reached out to Phanuel Koech's agent Eric Lilot and got an explanation for why he hasn't raced this year. He had some pain in his fibula following a cross country race in January. It was eventually diagnosed as a stress fracture and he spent the next few weeks/months getting treatment. He's back training now and the plan is to race in July.
I also asked about Nelly Chepchirchir. He couldn't really give me an answer on that one but he said it's not a doping-related absence.