I think this is a bug and not a feature. I have four models and a metamodel, plus the signal hub all saved as a layout. Just now I saved that layout, then went and made some minor changes to each of the four models and saved those. Then I closed down QS191, then reopened it and came back to that layout, and all those minor changes I saved were gone... back to what the state was when I saved the layout. And then... I opened another copy of one of the models, and it has all the minor changes saved properly.
So a layout, rather than just being an arrangement of the windows on a page, is also saving a stale copy of the models on that page. If I don't notice that the stale model is stale, I may make another change and save this stale model over top of the real (non-stale, latest) model.
I didn't yet check to see if the metamodel too is stale. I also didn't check if it's just model parameters that are stale (like start and stop dates, and $amount per trade) or if the C# code is stale too.
Now this could be a feature, not a bug. I could see some utility in saving a layout that keeps absolutely everything just as it was at the layout save point. But that's totally non-intuitive, because "layout" infers that it's just a windowing arrangement thing, not the contents of the windows too.
So for now, until this bug is fixed or the UX adjusted to make sure we understand the behavior, just be sure and save your layout AFTER you've saved your models.