When I get two sims to get married, they become spouses. They get the marriage certificate and all. The second I enter CAS, they show up as "partners" instead of spouses. I can't edit their relationship unless I cheat it. The marriage certificate is still in the inventory, and their wedding shows up in their milestones. But the option to propose is there, it's like the marriage got annulled and it's just strange. Please fix it soon!
It is a known bug. You will find more information here
https://forums.ea.com/idea/the-sims-4-bug-reports-en/married-couples-become-unmarriedpartners-after-entering-cas/13524609
The Sims Team