This happened with me on two different occasions. The first was when one of the Sims in the couple was invited out with a friend, and I had her go without her spouse. The second time (different household, of course) it happened when I had my workaholic Sim "working in off hours" and with the at-home Sim I decided to pop into CAS. When I got back into Live Mode, the scenario ended. In both cases, the reason for the "completion" is that my Sims were no longer married (even though they are still married).
Same issue here, one of my sims went to the flea market while the other stayed at home and when the market loaded it came up with the scenario completed screen saying they were divorced. For reference they went on a date in a different world and returned at the same time with no issues, they also visited places together in their home world and returned at the same time with no issues. Has it been designed that they need to go everywhere together???
