This happened to me when I designed data structures for CD sectors, and used an outer struct to wrap all the different sector layouts (Mode1/2 CDs, ...). The short code below is more suited for a unittest:

    struct Outer {
        struct Inner1 {
            union { 
                Inner2 a;
            } 
            byte I_AM_AT_THE_WRONG_OFFSET;
        }
    
        align(1) struct Inner2 {
            ubyte x;
        }
    }
    
    void main() {
        writeln(Outer.Inner1.I_AM_AT_THE_WRONG_OFFSET.offsetof);
    }

This prints '0', which means that field 'I_AM_AT_THE_WRONG_OFFSET' wasn't correctly offset behind field 'a'. It works correctly when
a) align(1) is removed
b) Inner2 is declared before Inner1
c) 'a' is not inside a union block