I'm working on a 4 layers project - 2 signal layers (top and bottom), one internal ground plane and a "mixed" power tracks and ground internal plane which is the object of the question and that you can see below with a power plane like this:
In red, 12V, green 3.3V, black is ground.
There are no particular requirements for currents, we can consider that all tracks power small consumption ICs. 0-50mA.
Do you see a particular reason for this design vs "real" power planes roughly like this? I kept some black ground copper because there's an antenna strip on top layer at this location (yellow on the drawing.)