I also have this problem.
Tested it real quick. None of the welcome scenarios give me the option to choose a save. They just immediately go to the last save. Tested by creating an entirely new save, dropped a randomly made sim into a lot, saved and went back to the main menu. Then selected one of the Welcome Scenarios and it loaded straight into that last save, complete with the random sim I made.
So I exited back to main and tried some of the other scenarios. A pop up window came up and gave me a choice between New, Existing, and New Household in New Save. Not all the scenarios had all the options available, but the window still popped up and the option that wasn't compatible was greyed out. So the rest of the scenarios are working fine.
Even selecting one of the welcome scenarios in one of the other categories loaded up the last save. So it didn't matter which version of it I clicked.
May or may not be intended, but certainly potentially confusing, especially for new players who were (I thought) who they were targeting the welcome scenarios to. Still not convinced that confusion over the potential to have multiple households in the same save is not what’s behind this thread:
