When placing candles on a cake in creative mode, the cake appears to be eaten for a frame but then goes back to the uneaten state.
When placing candles on a cake in survival mode, if you have hunger, then the hunger bar will increase visually, until the hunger bar is updated (like by actually eating cake or taking damage) and the hunger will go back to what is should have been.
Environment
Windows 10 Pro 64 Bit, Java Version 8 Update 271
Duplicates MC-204058