When you say the fellowship amount was not designated for 2018 expenses, you are telling the program that the fellowship you received in 2018 is to be used for expenses you had in 2017 or will have in 2019. The program won't tax this amount because it thinks you are going to deal with it next year, or dealt with it last year. Then when you allocate a portion to room and board, that becomes taxable. Fellowships used for room and board are taxable.
I don't think this is how you want to report it, fellowships usually don't work that way.