Hi !
Sorry to hear this, It's the bug That I try to fix for awhile now, Look like the bug will keep coming up randomly if I not get rip of it for good.
For the temperary fix, please reload the save that is before the visiting fern step or wait for arthur in 2 days to make the quest skip thise part completely(I already code it to make the step skip go seeing fern and waiting for his research entirely). If the save is already in that part I afraid the quest won't continue.
Thank you for your feed back. And sorry for not good experience you get from the bug .😔