Currently, in 1.21.1, with Productive Bees 13.5.3, and Realistic Bees 4.0, if you use a productive bee spawn egg to "spawn" a bee from productive bees, if the "extraBeeSpawnsPerBee" is set to anything BUT 0, the Minecraft bee spawn egg works as intended, splitting into the set number defined here. However, when using a productive bee spawn egg, the productive bee in question spawns, but them splits into only 1 productive bee, itself, AND the rest are Minecraft bees.

Here is my suggestion: have an option or do not, but allow the mod to limit extraBeeSpawnsPerBee to only Minecraft bees, if possible, AND OR make it work properly with NBT data of the spawn egg / bee entity in question.

So, I learned something new: "split" is not defined and or used properly in your config file, at least not from the main meaning of the word. Currently, it refers to "additional" bees to split from the original bee, instead of having the original bee split into multiple new bees, which nullifies my point above, where when setting it to 1 split, it creates 1 !additional! bee, rather than splitting into 1 be (or just the original smaller bee). If you set split to 0, then only the original bee, but smaller, remains. However, the issue still persists that these productive bees do not split, they split into the main productive bee, and then multiple vanilla bees.