I cannot figure this out. Originally it is said to happen IF the city elf is romancing Leliana.
Then it is stated if the player is romancing more than one npc.
But in my game this happens if romancing Zevran or Leliana. AND it does NOT occur if romancing Alistair and another npc at the same time IF Alistair is in the party.
If NOT romancing Alistair (romance active with Zev, Leli or both) and Alistair is in the party, the bug does not occur. This could be because Alistair has a speak-line after Zevran or Leliana ask the warden about the wedding.
I have not tried a male city elf romancing Morrigan to see if it occurs for her too.