This might not be it, but I recently opened a scene ant two of seven forest objects were just showing the "F" icon and no matter what, they would not make any items.
After some fiddling around, I found they had a camera assigned to them under camera settings. Clear the camera and the forests behaved again. It was odd, because I don't typically assign cameras for these forests (e.g. that must appear in reflections if they are behind camera) and I don't have any employees that would have set it. also, my other forest objects were as I had left them, with the camera field empty.
Since i fixed it, so far it has not happened again, but it is a quick thing to check off the list.