Romantic Relationships decay/become negative fast with no interaction

Just today I noticed the two sims with a good relationship suddenly had their romance bar in the red. I thought I missed something, but then I cheated the relationship up to max and watched the romance bar drain in seconds. I double and triple checked their relationship status. they had a wholesome dynamic were definitely attracted to each other, lots of positive relationship sentiments. The romance bar just won't stay up and romantic interactions make little to no difference.