PUG raid was near the end, maybe 11/12. Suddenly the event reset to 1/12. Discussion in chat blamed the the leader for having invited another player and claimed that’s what caused the reset. I don’t think the leader changed.
Not sure what happened but I can assure you from chat commentary that no player enjoyed losing their progress and this window to complete the event (the raid immediately fell apart so there was not an option to continue.)
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)