Thanks for the reply. Any idea how long, e.g. is it a few days sometimes? It's been over 7 hours and the Zone Guide still shows that delve as not completed. I'm not too fussed about it right now, though, because I'm not trying to complete the Deshaan zone (yet).Sometimes they take a while to register.
I redid the delve yesterday and this time it completed properly. As you said, it's not consistent, but I was "third time lucky".I know usually it registers right away but it can take hours or more.
Don't worry about it and go on with other things. Should be fine in a day or so.
There are a rare few delves where there's multiple bosses you have to defeat. This one doesn't appear to be one of those.
EDIT: To clarify, it's still a bug. I would still use the in-game report mechanism when it happens, but this bug is somewhat of a 'known' one that isn't consistent. The more reports, the more likely it gets scheduled for a fix someday.