In the below line, you retrieve the second item from the list each time without changing the list. Hence the final collision bounds are only ever (at most) collision.get(0).union(collision.get(1)), which I assume is incorrect. Perhaps you intended to use collision.get(i) as that is what the i variable is counting.

https://github.com/dan200/ComputerCraft/blob/master/src/main/java/dan200/computercraft/shared/common/BlockGeneric.java#L246