I have a Synology NAS. It has the possibility to encrypt folders. I have enabled it via the web interface. But one of the reasons for actually encrypting these files is that they should be protected even if if someone steals my NAS. I have mounted the drive on the NAS via the web interface.
What happens when the power is cut and the NAS boots again? Is the folder automatically mounted again? Is the key stored in volatile memory, in flash memory or on the hard disk? If it is stored in volatile memory there should be no risk at all.
Compare it to an iPhone. If you enable screen lock, the key is thrown away when activated.