I've been revisiting spirit of steam recently on a series S and I've noticed that, even after spawning multiple times on different services, there is some rolling stock that never seems to appear unless spawn it in. These being the conflat wagons with the container on and the blood and custard mk1s. I've gone back through articles and posts to see if there is an obvious reason for this and I haven't been able to find anything. I've also had a similar experience for the Goblin line as I don't seem to get any Azumas on the ecml just the 700 and again I can't find anything that says this is how it should be on a series S
When I had Series S, I never saw any Azumas either. Now that I have Series X, I do see them. Unfortunately, there are several instances where layers are not present on Series S, with little or no documentation. SRM and Shap are two other examples. You may have discovered another one with SoS. I'm sorry but unless you upgrade there is no other solution. What I would say is that, most of the time, whatever is missing is nothing that major. However, my problem is, and always has been, that there is almost no way to know about any of this before you buy something. Good luck.
And even when there is documentation or a forum post from DTG about the layers, the information can be incorrect or outdated. For Kinzigtalbahn they spread incorrect information about stuff what should be there, but wasn't.
Another case I've just found is on BCC. I have GWE installed but the layer for BCC isn't available to activate despite restarting the game. This one can't due to platform restrictions as I could have these layers on Gen 8
