There's a lot of existing code that assumes OnCellLoad will always fire after graphics are loaded.

Once upon a time, I'd instrumented this. I'm sure it used to work upon loading a saved file. But I've re-activated my same old debug lines, and it no longer seems to work.

In vanilla, CarriageDriverScript (the furniture version) expects OnCellLoad() to SetRestrained() the horse. Not working now.

In vanilla, CartDriverScript (the cut version), also used it to attach the cart to the horse. This code was adopted by Real Carriages, Scenic Carriages, and Touring Carriages. Not working now.

The problem is that the game engine doesn't save these states (and a host of others), so they have to be setup whenever the game is re-loaded. And they won't work until graphics is loaded (they throw errors in the log), so OnCellLoad() should be a perfect fit.

I've also tested OnCellAttach(), and that doesn't trigger in loads of saves either.

It does seem to work by loading the game, fast traveling away, and fast traveling back.