Due to privacy concerns, Instagram does not provide us with any user data since December 2018. Specifically, the user name and profile picture are not available on the Instagram API for posts from Hashtag sources. Instead of the username, "Instagram user" will be displayed as the creator of the post on your wall. Furthermore, blacklisting/whitelisting in Moderation is not available, because we can not relate a specific post to an Instagram account.
Is there a workaround for this issue?
No, there is no workaround to circumvent this behaviour.
Does this apply to all Instagram sources?
No, this only applies to Instagram hashtag sources, when adding an Instagram profile source, the user name is displayed normally.
Can I still find out who is the author of a specific post?
Yes, you can click on the time stamp of a specific post. This takes you directly to this post on instagram.com, there you can see the name of the user who posted it.