from the manual:
"EXPORT FOO.HTML DAT1.P8 DAT2.P8 GAME2.P8"
not crystal clear, but I guess those 3 carts are just added alongside the currently loaded one, say GAME1.P8.
but then load("game1.p8") doesn't work from GAME2.P8

this only happens with the main cart, so it seems the "boot" cart name is not set properly.

here's a workaround:

--boot.p8
print("I am Boot")
load("game1.p8")

LOAD BOOT.P8 EXPORT FOO.HTML DAT1.P8 DAT2.P8 GAME1.P8 GAME2.P8