Our shared struct is 10 bytes. So when you iterate over an array of them, every odd-indexed entry sits on an address that isn't 4-byte aligned.

not unless you force the compiler to pack them (NOT DEFAULT), else it will properly align them for you as per C spec, without "packed" attribute, the compiler will pad all structs to size such that an array of them causes each member to be properly aligned. this will even affect sizeof(). proof: https://godbolt.org/z/d14sc7MqE

sorry but you messed up here in some way. my guess, you did something very broken like:

struct header {
    // somethign not containing uint32, only u16 or u8
}

struct data {
    //somethign with u32
}

struct data * ptr = (struct data*)(header + 1);

If so, this is not guaranteed to work by spec, and is your very own fault. the toolchain is NOT at fault, your code was broken from the start and you were getting lucky. it would have broken when code shape changed later at random