Warning: ignoring packed attribute because of unpacked non-POD field
This page covers ArduinoJson 5.13.5, consider upgrading to version 6.10.0.
If you pass the flag --fpack-struct to the compiler, it will generate the following warning:
warning: ignoring packed attribute because of unpacked non-POD field
No solution has been found so far: you need to remove that flag if you want to get rid of that warning.