Romance decays upon successful interactions

Ugh, I am absolutely losing my mind!

So I'm having some trouble with romances right now.
My sims are married and I spent a couple pf days having them focus on their career promotions so that I could get enough money to buy a bigger house so they could have a kid.

I checked their relationship though and the relationship bar has gone from maxed out to empty, so I've tried raising it. The only problem is that I'll raise it a little bit, but no matter how many successful romantic interactions I have, it'll almost instantly go down to nothing again.

I don't understand what's happening. This has to be a bug, right?