There have been sharding issues for quite a while now. For the last several expansions the system has been forcing people, individually and in parties and raids, onto other shards of the zone they’re playing in and thus losing progress on events, world bosses and rare kills attempts, quests and the like.
Link below to another forum post:
Have you been forced from a shard while remaining active? - Community / General Discussion - World of Warcraft Forums (blizzard.com)