Assigning struct word member from another struct word member

I have noticed that assigning a member variable (word type) of one struct from the word member variable of another (different) struct does not seem to work.

eg: If we have:
struct STRUCT1 { word a }
struct STRUCT2 { word b }
pointer.STRUCT1 pS1
pointer.STRUCT2 pS2

then
pS1->a = pS2->b
does not result in the correct value in pS1->a

Start with the reported Millfork example involving assignments between word members of different structs. Trace how struct member access and word assignments are compiled, then reproduce the incorrect value with the example and add a regression test showing that pS1->a receives pS2->b.

Written by the indexing model from the issue text.