You’re right - the nzbdrone user is not visible in the Synology UI, but it should show up in /etc/passwd, in the same way that the nzbget user does. I’m also using WinSCP to look at the permissions and had made a screenshot of them after the install - the owner should be nzbdrone and not 100.
I did some more digging though and it would appear that the user creation process is a bit strange on the Synology anyway, so I made the decision to back up the db and xml, uninstall Sonarr and then re-install it and then restore those files - a bit of a pain but probably easier in the long run and now the nzbdrone user exists again at least (hopefully it will survive the next reboot!).
Maybe you can help with the restore now though as I’m not sure I have done things in the right order.
My old version (and therefore my database) was from the November update of Sonarr (220.127.116.1154). The version that gets installed initially from the Synocommunity goes back much earlier to 18.104.22.16872. I installed that initial version and then restored my database file but now it will not update to the latest version. Should I have done it differently and installed the initial 22.214.171.12472 version, then updated to the latest (126.96.36.19985), then restored my database? Or will that cause a problem because it is actually a later version than my database?
I can’t see a way of updating to the November update - only the latest version.