The bug
minecraft:egg is not present in the Spawn Eggs tab in the Creative inventory. This is despite the fact that they are an item that can be used to spawn mobs. Other non-spawn egg items that spawn mobs are also in this tab (i.e. spawners, trial spawners, creaking hearts), so the exclusion of eggs seems unexpected.
How to fix
Add minecraft:egg to the Spawn Eggs tab.
Unlike a spawn egg, chickens only have a 12.5% chance of coming out of an egg, as compared to 100% from a spawn egg. In any case, since there is already a Chicken Spawn Egg, this seems to be working as intended.