I realize now that this post should be merged with this thread here:
In that thread, @SalixCat details exactly what happened to me. It was a hangout at the cafe during the current STS that triggered the missing relationship.
Update: I did indeed find my missing Yamachan 1 day later, much to my surprise, walking around outside my main lot. I now have to re-do a relationship with him and re-marry, etc. to bring him back into my household.
Noted: Do not do hangouts at the cafe anymore.
I really hope there can be a fix for this soon.
