My reading is that this is entirely Go’s fault. In Go, there was a union of two different types without metadata to distinguish the two. The disambiguation was done by looking to see if the data looked like a certain pointer.

I think the reason it is a problem only in 32 bit systems is that the assignment setting the lower half of the data is a 32 bit op, which can overflow the 16 bits used, but won’t affect the other half of a 64 bit value. (Corrections welcome.)