Married Sims keep getting demoted to "partner" status any time I enter CAS?

My single Sim married a townie - had a whole wedding party and everything, they got married at a wedding arch and the game prompted me to move the townie in... so I'm pretty sure they did get "spouse" status initially.

I played for a couple of Sim days and noticed that I had the option to make them propose to each other again. I was a little confused but figured it was a one time glitch, got them engaged again and had them elope. 

All seemed well for a while. They had a baby and as soon as their daughter went from infant to toddler, I used CAS to change her outfits. As soon as I left CAS, it told me her parents had become each other's partners and... lo and behold, they're no longer married, but instead only dating. Now I'm realising this is what happened the first time as well - when my Sims married, I entered CAS to change the spouse's surname, so it must have been CAS that got rid of their married status. Is there any way to fix or bypass this? Like I'm happy they love each other but getting married three times is kinda excessive so I'd rather they just stay married for good!!