Thanks siladris and HollownessDevour, what you've said has made sense. Interestingly though, the couple I mentioned above are part of a rotational save and while playing the other household in the save I got the "I found a ring in my partner's bag" pop up. I choose the "do what you think is best" option and when I returned to play the couple they had become engaged 'off-screen' but still had no romance satisfaction. I guess because I wasn't playing with them when they got engaged the romantic satisfaction didn't trigger when their romantic status changed.
It's just dumb and annoying that this is the case for pre-existing couples, especially married ones. It shouldn't be necessary to cheat to lower their romance level to get romance satisfaction once it goes back up, purely because there are no romance statuses left after marriage to trigger satisfaction! I really hope this wasn't by design.
The Sims Team