When my first Monarch met the possible people he could snoggle with at the start of the game, he chose himself some bard man-lovin' -- he was lusty and having a passel of ** kids seemed unwise for the health of the kingdom.
Well, when I selected "Ask to Court" I thought it meant the ROYAL COURT -- in other words I thought i was inviting him to the throne room (maybe to look at some etchings?) but instead I got a boyfriend! Whoops! So much for lack of commitment Monarch.
My second hero was a bard and the SECOND I got him the Monarch's Bard Boyfriend kept hiding out behind the scenes (I guess he hated the musical competition?) -- the only way to make him appear was by sending him a carrier pigeon to invite over. We'd have a good flower presentation and kiss and that would keep the lusty edge off, though sometimes we snuck upstairs to the double bed if I wasn't needing to pass a pressing edit.
Long story short: It seems any NPC who gets in a commuted relationship with your PC bugs out and you have to invite them over. Mind you, this makes cheating and getting away with it a lot easier. I did notice there is an INVITE TO HOUSEHOLD interaction, perhaps if you haven't done that, that may be the cause of them having to "go home" to elsewhere. . .