Can't see collaborators anymore#1383

I shared a list with someone and at first I could see them and assign tasks to them, but now for some reason I cannot see them on the list and can’t assign them to tasks. On their end, they can see that they are a part of the board and the tasks I was able to assign to them previously. I tried sharing the list with them again, but they just get an error saying they are already part of the list. I am confused why they can see they are a collaborator on their end and I can’t see it on mine. Please advise.