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 requested a position sync. The Scout should be back in your unit later today. You will need to place him in a den in Scoutbook after he is back on your roster.
I recommend contacting your Council and asking them to end the membership in the Scout’s previous pack.
I do see a bug in Scoutbook that caused this. I will notify the developers.