The CIRCUITPY_INTERNAL_FLASH_FILESYSTEM_SIZE variable adjusts how much of the flash is dedicated to an internal filesystem, and changing the size causes the filesystem to move. Since it's located in a platform header file, it also causes a rebuild of most of the port.
However it does not cause a rebuild of the linker script, meaning in practice the filesystem is physically positioned at the old location. Additionally, pointers inside circuit python will not match the intended layout.
common.ld should depend on CIRCUITPY_INTERNAL_FLASH_FILESYSTEM_SIZE, which probably means other files upstream also need to depend on that file.