Here is a screenshot, which says more than 1000 words i hope.
Have 18 Quests in Journal, showing all Quest-Names by the API, it returned "blank" for one. You can't get any informations any more for that quest, with any Quest-API out here.
You also can't share this quest any more.
I have had this in the past, it happened 2 or 3x during the current NewLifeFestival,
2x on a delve-daily quest in the last blackwood-event.
Reported by others it happens much more often to them, but have no verification for this.
You can still complete the quest ingame for yourself without any issues. You get the rewards, all fine.
You can just not share them, and i guess they break any addon which deals with quest-names.