If this issue happens on a project that was created by converting a Creative Island a while ago during the early stages of the UEFN alpha, this might fix your problem:
Unfortunately I have the same problem which only started on the 8th of July this year and it is a map created with UEFN over the last 2 months, hoping it can be fixed as a lot of work has going into the map so far and it still is not finished yet.