Not by default. It remembers the MAC for each specific network. This is because sometimes you want to have specific device on the same IP all the time. The DHCP decides this via the MAC.

If you want it truly randomised every time you need to turn on non persistent MAC randomisation in the developer option.

Are you unpacking them directly on your NAS?

Because chances are, that if you use a Synology and map the drive or use the GUI, that the data keeps getting transferred between your PC and the NAS. This is a very slow process, especially if you connect by WiFi.