The export command and the -export command line option do not flatten #includes when exporting a .p8.png file. A cartridge exported from a .p8 program that uses #includes cannot be run.

For .p8.png use save instead of export.

save mycart.p8.png

Yes. The drawbacks of that are (a) that it is easy to forget to switch back to your original source, and then any edits you make are saved into the PNG, and (b) you cannot save the cart to a different filename with a command-line option, meaning that you cannot automate the build of the final cart PNG.

Export for PNG cart is recent, this is probably just a bug.