This is not a bug. It is part of the DnD rules:
As per the multiclassing rules on page 164 of the Player’s Handbook - “Unarmored Defense. If you already have the Unarmored Defense feature, you can't gain it again from another class.”
Luckily, you can swap between which version of Unarmoured Defence you utilize. Whichever stat is highest, will be the one that is utilized.