This bug was fixed in Patch 1.82.99.
My Sim baked a cake. Unfortunately there’s no option to add a wedding topper or birthday candles.
Cakes which are baked in groups now have 12 servings instead of 8. Unfortunately that probably leads to the game no longer recognizing the cake as valid to add candles/toppers.
Bake a cake with only one Sim. This cake still has 8 servings and is correctly recognized by the game to add candles/toppers.