I think it's in the API used but I'm reporting it here as well.
All the info is in this issue on the API:
kyrptonaught/customportalapi#32

Oh, my bad! I was browsing some other issue and I understood the resolution as "the problem is not in this mod but in the API" so I thought the API was related but it instead said it wasn't related.
Oops!

Custom Portals does not use Kyrptonaught's customportalapi, and the two mods are unrelated. Their similar names are purely coincidental.