Comment by FireBeyond

> for the simple reason that opening new instances of apps is inconsistent and often doesn't behave how you'd expect it to once one more than one space is involved

System Settings > Desktop & Dock "Automatically rearrange Spaces based on most recent use". This is the critical part.

And then right click App on the Dock, Assign to this Dock.

With these two things, Spaces becomes predictable and repeatable.