Unable to Mount LittleFS File System On External Flash On Thingy53

I am developing with thingy53. I would like to enable BLE function, MCUboot and littlefs file system at the same time. I am using SDK and toolchain v2.7.0 and sysbuild as my build option. From the post here, I understand that one can use 

CONFIG_PM_PARTITION_REGION_LITTLEFS_EXTERNAL=y
CONFIG_PM_PARTITION_SIZE_LITTLEFS
to configure mounting on the external flash and its size. So I added these two lines in my prj.conf along with other necessary configurations needed for file system. The project could be built without any problem. The file system is indeed now on the external flash, but when I examine the memory report, I found that the size of the external flash overflows, see attached. 

  This clearly shows that littlefs_storage is located between 0x00800000 and 0x00805fff when the external flash's max size should be 0x00800000 already. How can I solve this problem??

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