It's a bug. Changing rendering systems usually solves it. If it doesn't, there was some other way to fix it but I've long forgotten it. Hopefully the rendering change fixes it.
I've noticed this happen over the years too, and it seems to be one of FH's "nope, you added too many things to your game, I'm going to stop this function now" kinda things. Sometimes people have modded their game or added maps to their game and had this happen, which made FH's size bigger. I don't know how it effects this exactly? But it does? Sometimes people just use Direct3D9, or remove things from their game and works again. I know the Direct3D9 thing has been mentioned before, but I just wanted to generally post on what I've noticed happen over the years.