Solaria had two duchies. One contained a city, the other a stronghold. Sun Hall aligned with Luria Nova, leaving Solaria with a single duchy and a stronghold. Remember: in the new political model, there is nothing special about a city except that it makes a lot of money and is one of two region types that can be capitals. Strongholds are the other. This has never been a bug, unless we redefine bug to mean something that has been explicitly allowed and not coded to prevent. The merger of the second duchy into Luria Nova should not have been allowed, because the code is supposed to prevent this from happening when a realm is left with a single city/stronghold. It was a behavior that was not expected, given the parameters in the code.
Please be mindful of how the word "bug" is used. In part, because it means something very specific to the code, which is not publicly-accessible. Also because you open the door to the silliness of defining behavior that one might find personally disadvantageous, or not properly understood, as a bug. BattleMaster is littered with examples of conventional wisdom about certain things being completely wrong because someone said X and everyone agreed.