It appears to be a bug on the PICO-8 end, introduced in a recent update. It affects most of my carts and I'd have to work around it by changing the carts one by one, so for now I contacted Zep (the creator of PICO-8) to see if I can get it fixed in a future version.
(EDIT: zep tells me that the bug will be fixed in the next PICO-8 version, 0.2.1.c)
This cart will work fine in any 0.2.0 or earlier version (you can get those at https://www.lexaloffle.com/games.php?page=archive)