As you play with a couple that's married with great compatibility and attraction, and with a couple with bad compatibility too, the romance bar reduces very quickly. I could sit and watch it go down on its own within the matter of minutes. It even turns fully red despite there not having been any issues or fights. It happens a lot in just a few in game days. The bar goes fully up to in pink with just a few interactions too. I don't know what patch caused this issue but the red bar is extremely concerning.
Hopefully it's an easy fix! Thank you for investigating when you do!