I've noticed that when I try to build a makefile for multicarts that calling export from the command line of my host machine doesn't respect root_path.

For example:

pico8 -root_path ./ -export mygame.html title.p8 intro.p8 mygame.p8

produces an html export that only gets title.p8 but neither of intro or mygame carts if the root path isn't already the pico-8 cartridge directory.

Does anybody know of a workaround short of actually changing the pico-8 system config for each new directory?

filled a bug some time ago on that and was supposed to be fixed:
https://www.lexaloffle.com/bbs/?tid=40098