The times I had issues in any daw, just as closing project, was due to some plugin that messed up memory.
Try the Save copy to(or what it's called) and see if to do with actuall instances in project, or something else.
The load project, play back a bar, and close - and see.
Freeze half of synths at a time - and see if different.
If not unfreeze and freeze other half - and see...
Using new features like Mix recall - could be a source to trouble if changing a lot of plugins between saved mixes.