Welcome! This forum has a treasure trove of great info – Scouters helping Scouters! Just a heads up, though - all content, information, and opinions shared on this forum are those of the author, not the BSA.
I have two new-ish scouts that aren’t showing up in SB+. Member ID 141940863 and 141643842. They show up in the my.scouting roster.
I also have a scout 142034089 showing up as unassigned. I’ve assigned her to the Lion den and saved. But when I go back to it later, it puts her back as unassigned.
Are you one of the Key 3 or a Key 3 Delegate? With the new permissions structure, I think you need to be in one of those roles to assign a scout to a den. There might be other specific permissions that could be assigned (Profile Editor, maybe?) that could get you there, but it would probably need to be assigned at the unit level, not the sub-unit (i.e. den) level. Again, the Key 3/Key 3 Delegates can assign those permissions, I believe.