if I add an SSH key in Profile → Login and Security → Advanced Settings …
it will no survive a reboot.
Best to open a ticket, QTS 6 is only RC and not fully final
May we ask if you have not yet created a home folder on your NAS? If that’s the case, the settings would have nowhere to be saved and would be stored in RAM disk instead, which means they would be lost after a reboot. We will also be planning to improve the related prompts in a future version. Thanks for the report!