Hi @LiveDuo, currently callbacks are registered using 1 function pointer and 1 env pointer per callback. The rust-cdk abstracts over that by pinning the Future/Waker and then registering the callback with the function-pointer as the global-cdk-callback-function, and with the env pointer as the pointer to the Future/Waker pin. When the callback comes in, the global-cdk-callback-function uses the env pointer to find the Future/Waker pin of the original Future and wakes up the future, thus continuing from the await point.
In this case, the global-cdk-callback-function in the new module after the upgrade has a different function pointer value than the pointer that is registered for the callback and the new module does not contain the Future/Waker Pin of the original Future because it was not carried over through the upgrade, so the function pointer and env pointer are invalid.
One coming feature on the roadmap is the name-callbacks feature, which lets canisters register callbacks using function names (instead of pointers), so that callbacks can come in across upgrades as long as the new module contains a callback function with the same name as the registered callback. The name-callbacks feature lets canisters upgrade themselves in a safe way and without stopping.