Hello @JetWen,
It seems that your issue has something to do with your memory configuration.
The MKL26Z256VLH4 has different memory values than the MKL26Z128VLH4, so it might be a good idea to modify your memory configuration for the new device.
[KL26P64M48SF5. Ordering Information]
[KL26P121M48SF4. Ordering Information]
Just for double check, could you please check and modified your memory configuration if needed?
Warm regards, Raul.