Nothing bad except for someone just flooding your blueprint storage. Just seemed like a bug especially since it only happens when you hold a blueprint item not when you drag a blueprint from anywhere else

You can not delete or modify anything in another user's blueprint library which is why I found this odd. Either way, if you need someone to troubleshoot your blueprints there is still the option of them getting it from your blueprint library or just putting the blueprint onto the ground.

If you click on another player's blueprint library while holding a blueprint, it will put your blueprint into their library. These blueprints are visible to both you and the other player and stay even after disconnecting and reconnecting. It should be saying that I can not modify another players ...