Unable to filter woocommerce_package_rates

I am no longer able to filter package rates, and after further debugging, I discovered this is due to the following conditional check failing in includes/class-wc-shipping.php:

if ( ! is_array( $stored_rates ) || $package_hash !== $stored_rates['package_hash'] || 'yes' === get_option( 'woocommerce_shipping_debug_mode', 'no' ) ) {...
on line 332

So none of the actions or filters under this are working. The $stored_rates is an array, so the first condition fails, the second condition fails because the hashes are equal, and the last condition fails because I do not have shipping_debug_mode enabled.