https://wiki.mikrotik.com/wiki/Manual:System/File
Warning: If device has a directory named “flash” in its file list, then files which you want to be kept after system reboot/power cycle must be stored within it. As anything outside of it is kept within a RAM disk and will be lost upon reboot. Note: this does not include .npk upgrade files as they will be applied by upgrade process before system discards the RAM drive content.