Given that everything seemd to be working properly until Determine Offered Salary exercise, once I completed that exercise, the cases all end after Perform Background Check (which did display the background info).
If I change the the upon stage entry for the Assess Candidate step from upon stage entry to upon completion of previous step, the cases run through the Offer and Wrap Up stages to normal case resolution.
Any ideas what might be causing termination after the background check?
This is due to a known product bug: essentially, if you run a flow in draft mode in this situation, and the flow contains a decision shape that doesn't resolve to a decision rule (so that you have to select the outcome), case processing stalls out.
There is a hotfix available to resolve the issue, or you can just create the decision rule with the allowed outcomes to avoid the problem.
What version of Pega7 does your exercise system use? And are you using the PVS or a multitenant system?