I don't know if this is good or bad or needs to change, but I have noticed a funny quirk of the colony/core-world system. Often, when I convert a colony to a core world, my productivity goes down - in some cases, by a lot. I have noticed this in particular with food, but the same effect will happen with all the "primary" planet products (i.e. research, food, wealth, etc.).
The reason for this is that the primary products which are fed from colonies to core worlds are then multiplied by all of the relevant modifiers on the core world. (This includes improvements, citizens, etc.) So if, for example, I have a class 13 colony ("Bob") supplying 8 food to a core world ("Alice") with tons of ag districts on it, that 8 food might have a 2x multiplier. If I then turn that colony into a core world, then I no longer get that multiplier.
Even more extreme, the new core world also siphons colonies away from the old one. So I don't just lose that 2x modifier for food from that particular colony that I converted. I actually lose the 2x modifier for *all colonies* that switch from Alice to Bob.
I don't know if this is intended. It's certainly unexpected as a player. I don't think we can fundamentally eliminate this effect, though, without totally busting the colony/core-world system.
One thing I think you should consider is *not* automatically reassigning colonies from alice to bob when bob is promoted to core world. Maybe it should work more like shipyards did in GC3, where we can select the colony assignment.