Cake can be eaten in adventure mode

I can confirm this behavior. This may be by design judging by the fact of how you're able to eat other foods in adventure mode such as steak and bread.

Eating cake you actually "break" it, and steak and bread you can eat without breaking blocks 🙂

Yes, I do see where you're coming from here, nonetheless. Cakes can indeed be placed down as blocks whereas steak and bread can't.

Block interaction isn't prevented by adventure mode, only placing and mining. Cake being eaten is interaction, not mining.
Item usage on blocks changing said block is controlled by CanPlaceOn (that really needs to be renamed/split off), so stuff like candles not needing it when placing on cakes is a valid report. Eating cake doesn't require an item, isn't mining and isn't placing, so it matches the limitation of adventure mode as designed.

There are many interactions that are prevented by adventure mode (eg MC-184778, MC-204707). There are also many reports that describe adventure mode interactions (MC-222818, MC-222682, MC-222494, MC-222405, MC-222404, MC-213954).

Maybe Mojang should just add a block tag that lists blocks whose item-less interactions get disabled when in adventure (would include stuff like candles, redstone wire and stuff like that but not levers, buttons, doors), that way if you want something to be disabled/enabled, you can actually choose to do that.
Cake is rarely used as a build part (unlike candles, and redstone wire straight up changes the behavior when toggled), and much more as actual food source.

As far as I know, only the candles and redstone wire item-less interactions are disabled in adventure.
The second list of reports are actually item interactions that don't need CanPlaceOn, which is a different category of issues.