I've looked into this.
This problem doesn't occur with the latest recommended version (10.13.4.1448) which is the one you should use.
It seems that the creators of Forge have once again done something they 'think' is correct, and litterally screwed over lots and lots of modmakers in the process (they like to do this, and tell us we were wrong),
you're going to have to wait for an update from me before you can use the mod on those later versions, I am very sorry for this inconvenience.