User Problem

The assigned of a card is removed while moving a card with a member assigned to a new board if the user isn't a member of the other board. 

Suggested Solutions

It'd be nice to have a message explaining that the user isn't a member of the other board and if you want to add them to keep them assigned on the card, not move the card or move the card without the member. 

Current Workarounds

You need to manually check if the user is a member of the other board and add them before moving the card.