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 1, 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.