I had two teens who exchanged promise rings. As young adults, they had all the positive friendly/romantic relationship states but the Propose option was grayed out.
Asking to be "just friends" and rebuilding their Romance bar was very fast since their friendship bars were maxed. They could then Propose.
This is not the same issue as you have, but I share my experience in case the solution works for you.
Maybe you could make a copy of your current file and test if that will work for you? If it works in your copy/test file, you can go back to your "real" save file.
Good luck!