I didn't think to check on that, so I just did. It now appears to be only this project. Maybe it has something to do with portals? I opened the precursor project to this one. I got that one to a result I liked, so I saved it as a new project. I hope the following won't be too much detail but this is the best I can remember. This was days ago.
In that original version, I experimented with the "Move to new graph" feature.
I had two texture graph tabs going, and the one terrain graph tab, so the terrain graph ended with an Erosion node set up as a portal.
I remember there was some wonkiness while trying to work on the snow texturing. I clearly wasn't doing something right.
At one point I was fiddling with those portals, trying to break their connection, but it wasn't working.
I exported the terrain graph, and then also the Spring look texture graph.
I may have started a new project, deleted the two default nodes, and imported the terrain graph (I'm not super clear about this. I was working late at night, and may have been tired)
I'm pretty sure I rebuilt my texture graph connected to the terrain graph in the same tab.
It was at this point where I noticed, after closing Gaea, and re-launching, that the terrain side of the graph was now hidden.
If I dig into the autosaves, I might be able to find the first version where it started happening.