The context menu for a field needs an "Update" entry, which currently it doesn't have. This is obvious when fields aren't auto-updated (Configure this via the menus: Tools | Options | LO Writer | General | Auto-Update) ; but it is also relevant when fields do get auto-update... in case of auto-update failures, e.g. bug 113767. I _think_ we've never had this context menu entry, correct me if I'm wrong.
Current Tools > Update Fields work on all fields and is not suited for a context menu. We first have to introduce a command to update the selected field only. This request has been filed by yourself before in bug 148979. *** This bug has been marked as a duplicate of bug 148979 ***