750GR3 autoclean files

Hello, everyone. In my brand new 750gr3 happens weird things. All files in directory “files” disappear each reboot. I have several 750gl and a lot of rb2011 - these are fine.
I use RouterOS 6.40.1. Guys, help me plz.

This is absolutely normal. This model has a very small FLASH device and therefore the place where you can upload files is implemented as a RAMdisk.
It is lost every time you reboot. There is a subdirectory “flash” which points to the FLASH device and files you put there will survive a reboot, however there is barely any space.
When you want to have more space you will have to add an SD card or USB memory stick.

thanks a lot!!!