I've been experimenting with the new romance system and I've been finding odd issues with it. The problems seem to stem from being already married. You can give people lockets to date them, but they're no longer registered as your partners, they stay marked as friends, but you can hug them.
Prior to the update I already had Yuri as a girlfriend on one save file while also being married , so when giving her a new locket I was promoted with 'you are already dating', but on the hangout menu I couldn't do any romantic hangouts with her and the menu said 'you can't because you are not dating'. Quite the paradox there.
Then when progressing with Leah to 10 hearts on another save file, while again already married, her 10 heart event did not show up. It appears the only way to fully test the new system is to either not be married or start a new game.