TFM_BOOT_MPU_PROTECTION is defined in platform/ext/target/stm/stm32l5xx/boards/nucleo_l552ze_q/boot_hal_cfg.h. It configures and enables both a secure MPU and a non secure MPU for the nucleo_l552ze_q.
It can cause an issue for other non secure applications. For example with Zephyr OS, Zephyr should be the only one responsible for configuring the non secure MPU, but it currently cannot overwrite it.