This is very poor form indeed, however, bear in mind that Etihad fully divested from Air Serbia a few years ago, and the latter only has a token participation in Etihad Guest until they launch their own FFP in 2025. I'm not saying that this excuses the nonsense situation you encountered, but it does provide some context as to why Air Serbia is completely unmotivated to adapt its systems to facilitate Etihad Guest upgrades...