Don't know if it still applies, haven't updated BOP again today, but I noticed when I teleported to one of these biomes with the command to check it out the other day, if you placed dirt(just vanilla dirt!) next to the long grass, the grass that grows on it is origin grass, not long grass. i.e. the grass that's supposed to grow on Origin Islands.

Should be fixed as of 0687fb4
Try updating to the latest version and see if this still happens :)

I updated, now just to literally watch grass grow until something happens, haha.

EDIT: Nope, it still spreads into Origin Grass.