Trying UBSAN and ASAN might have been worth it. I don't think comparing debug and release builds would have helped: could be buggy optimisations or UB in your code regardless of what the outcome of that test was.
1. You check in both places in code in the debugger that the address of the field matches. (Maybe the object is at the same address, but due to some build system level ifdef mismatch, you effectively get different struct definitions in different places or something like that.)
2. You add a breakpoint inside the procedure and outside it.
3. When the breakpoint inside is hit, you add a watchpoint at the address of the field. Continue. See if the watchpoint gets hit before reaching the second breakpoint.